Peak week is when order limits stop being an operational preference and become a fairness mechanism. Stock is finite, demand is concentrated into hours, and the buyers most willing to work around a rule are the ones your rule exists to stop.
This is a checklist to run before the traffic arrives, not during it.
1. Decide what the limit is protecting
Two very different goals get called the same thing. A margin limit stops orders that cost you money to ship. A fairness limit spreads scarce stock across more customers. They need different rules, and conflating them produces a cap that does neither well. If you have not settled this, minimum order quantity or minimum order value is the shorter read.
2. Check the rule holds where the cart is skipped
This is the one that bites during a drop. A limit implemented in theme code is undone by three ordinary things: editing the quantity in the cart drawer, opening a cart permalink with a quantity pre-filled, and Buy Now, which goes straight to checkout without touching your cart page.
Shopify's cart and checkout validation documentation describes the layer that holds: functions that "run on Shopify's servers" and provide "reliable enforcement of business logic that can't be bypassed by customers", covering "online store carts, carts built for custom storefronts, and throughout the checkout process". Since 31 January 2024, that is available on any plan through a public App Store app.
3. Know the surfaces your rule will not cover
Being precise here saves an unpleasant discovery on the day. The API reference lists the unsupported surfaces: the Create Order API, Order Edit in the admin and in checkout, POS, Pre-order and Try Before You Buy, and Subscriptions for recurring orders. Draft orders are covered, per the changelog of 11 December 2024.
If you run a pop-up on the weekend, POS sales are outside the rule. Plan for that with allocation rather than software.
4. Count your function slots
The same reference notes that "you can activate a maximum of 25 validation functions on each store". If you install several apps in October that each add functions, check the total before peak rather than during it.
5. Announce the cap before the cart
A limit discovered at checkout during a drop is the worst version of the experience: the shopper has already invested and now feels tricked. Baymard Institute's research puts the average documented cart abandonment rate at 70.22% across 50 studies, with 17% of reasoned abandonments blaming a checkout that felt too long or complicated. Peak traffic amplifies every point of friction you have not pre-empted.
Say "limit 2 per customer" on the product page, where it reads as fairness rather than failure.
6. Test the violations, not the happy path
Before the sale, deliberately break each rule: over the maximum, under the minimum, off the increment, via Buy Now, via a draft order. Four minutes of trying to defeat your own store is worth more than any amount of configuration review.
7. Write down what you will relax
Some caps should loosen once the rush passes, and nobody remembers which ones in December. Note the rule, the date, and the reason now, while the intent is fresh.