When a variant goes out of stock, Shopify blocks the purchase — you lose the sale and never see it. With Substitutions, you pre-approve a backup per product. The app keeps the original sellable, then swaps in your backup at fulfillment. Customers pay their original cart price. When backups run out, the variant returns to out-of-stock and stops accepting orders. Built for CPG, subscription boxes, and B2B brands with substitute SKU agreements.