Guidance first, enforcement second
A quantity rule should not arrive as a checkout surprise.
A case-packed product, a limited release, and a wholesale item can all need quantity controls. The arithmetic is simple. The customer experience is not.
If the only explanation appears after the shopper has built a cart, a valid business rule feels like an error. Put a short hint near the quantity selector and in the cart, then use checkout validation as the source of truth. The first layer helps people choose correctly; the second prevents an invalid order from slipping through another path.
Write the requirement before configuring it.
“Customers must order this product in multiples of 6” is a rule. “Wholesale quantity control” is only a label. A precise sentence makes the target, number, storefront message, and test cases much easier to verify.
Choose the limit that says exactly what you mean
Use a minimum when an order below a threshold is impractical, a maximum when access or inventory must be capped, and a required multiple when fulfillment works in fixed increments. Do not replace a multiple-of-6 rule with a minimum of 6: quantities of 7 or 11 would satisfy the minimum while still breaking the case pack.
