Built for scarce stock
Cap how many units one order can take
Prime MOQ caps how many units a single Shopify order can take, scoped to a product, a variant, a collection or a tag, and the cap can follow your real stock level. A Shopify Function refuses the extra unit in the cart and at checkout, for guests as well as signed-in customers.
A spare-parts retailer keeps three or four of every filter on the shelf. That is enough, because each line sells once or twice a week and the reorder cycle is short.
Then a single order takes all eleven units of one slow-moving line. The next four customers see it out of stock, the restock takes three weeks, and every service booking that needed that part gets pushed back.
A cap of 4 per order keeps the shelf usable for everyone. Shopify refuses the fifth unit at checkout, and the message says plainly how many are available per order.
The rule
Maximum 4 per order
- Limit type
- Quantity
- Scope
- Collection: service parts
- Maximum
- 4 units
- How to count
- Each item separately
- Checkout enforcement
- Always on
In the app this is the Maximum quantityquick-start template, already filled in.
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Live demo - this is exactly what your customers see.
- Included
One buyer cannot take a whole line, so stock stays spread across your customers.
- Included
The cap is per order and applies to guests, with no customer account required.
- Included
Add a repurchase limit on the same rule when a second order would undo the cap.
Merchant profile
A spare-parts retailer serving independent garages and home mechanics. The catalogue is wide and shallow: thousands of lines, each selling once or twice a week, each stocked three or four deep because that is what the sales rate justifies.
The business is not built on volume per line. It is built on having the part, today, for whoever needs it, which makes availability the product rather than price.
The challenge
Nothing stops one order taking every unit of a line. A buyer who wants all eleven filters can have all eleven, and Shopify will thank them for the order.
The consequence lands on everyone else. The next four customers see the line out of stock, the reorder takes weeks, and every service booking that needed that part is pushed back behind it.
What they needed
- Keep a line available to several customers rather than to whoever arrives first with a large cart.
- Apply the cap without requiring an account, because most buyers check out as guests.
- Let the ceiling follow real stock, so it does not have to be revisited after every delivery.
Why it costs money
A stockout on a slow line is not a lost sale, it is a lost customer relationship. The mechanic who could not get the part today books the job elsewhere, and the next part goes with it.
The single large order looks like the best day that line has had all month. What it actually bought was four disappointed customers and three weeks of an empty shelf, none of which appears on the same report.
Approaches that fail
Hiding stock levels so nobody knows there are eleven
It changes what the buyer knows, not what the cart allows. Someone who wants a large quantity will simply type it, and hiding the number also costs the honest buyer the information they needed.
Restricting purchases to account holders only
It stops the problem by stopping the traffic. Most buyers on a parts catalogue arrive from a search with a part number and no intention of registering, so this trades a rare loss for a constant one.
Setting one global cap across the whole catalogue
A number that protects a slow-moving filter will strangle a fast-moving consumable. One cap for everything means it is either too low somewhere or too high somewhere else, usually both.
Weighing the options
| Approach | Protects availability | Works for guests | Survives a restock |
|---|---|---|---|
| Hiding the stock level | No, the cart is unchanged | Yes, but it helps nobody | Not applicable, nothing is enforced |
| Accounts required to buy | Partly, at the cost of traffic | No, that is the mechanism | Yes, but the cost is permanent |
| A maximum scoped to a collection | Yes, per order and per line | Yes, evaluated on the cart | Yes, especially if it reads stock |
The rule they chose
A maximum of 4 per order on the service parts collection, counted per item rather than across the cart. Counting separately is the deliberate part: a garage buying one of each of six parts is a good order, and a cap on the combined total would punish it.
Where a line is genuinely scarce the ceiling reads current inventory instead of a fixed number, so it tightens as stock falls and relaxes after a delivery without anyone editing the rule.
What the buyer sees
The cap is stated rather than merely applied. A buyer who tries for eleven is told that four per order is the limit, which is information they can act on immediately.
Because the rule is evaluated on the cart, it holds for the guest who arrives from a search with a part number as firmly as for a registered trade account.
What could go wrong
- RiskA legitimate trade customer needs more than the cap allows
- MitigationScope the cap to retail-facing collections, or tag trade accounts and exclude them, so the rule protects the shelf without blocking the accounts it was built for.
- RiskThe buyer places four separate orders instead of one
- MitigationAdd a repurchase limit on the same rule, which counts order history rather than the current cart and closes that loop.
- RiskAn inventory-aware cap turns hostile when stock is very low
- MitigationPair it with a floor so the cap never falls below a sensible minimum, and check what the storefront says when only one unit remains.
What to take away
- 1
Availability can be worth more than the largest order, and a cap is how you choose which one you are optimising for.
- 2
Count per item rather than across the cart when the catalogue is wide, or you punish the multi-line orders you actually want.
- 3
A cap tied to inventory keeps working after the restock; a cap tied to a number needs someone to remember it.
Questions
Questions merchants ask
- How do I stop one customer buying all my stock on Shopify?
- Set a maximum quantity rule scoped to the product, variant, collection or tag you need to protect. A Shopify Function refuses any cart above the cap, so a single order cannot clear a line. Pair it with a repurchase limit when you also need to stop the same buyer returning for a second order.
- Can the cap follow my real stock level?
- Yes. Instead of a fixed number the maximum can read your current inventory, so a line with 11 units in stock can be capped at a share of what is actually there. The cap moves as stock moves, which means no one has to remember to edit the rule after a restock.
- Does a maximum order quantity work for guests?
- Yes. The per-order cap is evaluated on the cart itself, so it applies to guests and signed-in customers alike and needs no customer account. Only a repurchase limit, which counts a buyer's history across separate orders, requires the customer to be identifiable.
- How are multiple items counted against the cap?
- You choose. A rule can count each item separately, so every line gets its own ceiling, or count the combined total across everything the rule covers. Counting separately suits a shelf of unrelated parts; counting together suits a collection where the total leaving the warehouse is what matters.
- What does the customer see when they hit the cap?
- A message you write, in the language of the storefront they are browsing, placed where you put the app block. It states the limit rather than simply refusing, so a buyer who wanted eleven units knows that four is the ceiling and can order that instead of abandoning the cart.
More case studies
Five other stores, five other rules
Every quick-start template in the app has a story behind it. Here are the rest.
Minimum order quantity
A beverage wholesaler stops buyers breaking a case of 6.
Sell in pack multiples
A candle maker makes the box and the order the same thing.
Minimum order value
A B2B food supplier refuses carts that lose money.
Weight limits
A tile store keeps carts inside what the carrier will take.
Limit quantity per customer
A collectibles brand stops resellers coming back for more.