Backorder

The customer pays now. You ship when the delivery lands.

How Shopify does it

There is a setting on each variant called “continue selling when out of stock”. Turn it on and the product keeps selling past zero.

Shopify does not track the backlog for you. It lets stock go negative and leaves the chasing to you, which is fine for one line and unmanageable for forty.

When it is worth it

Backorders make sense when you know the lead time and can say a real date. “Ships 4 March” holds a customer. “Currently unavailable” sends them to a competitor.

They also save a sale that a plain stockout loses outright, which for a product with a long reorder cycle is most of them.

Where it goes wrong

The date slips and nobody tells the customer. A backorder is a promise with a date on it, and the support cost of a silent three-week delay is far higher than the margin on the order.

The other one is leaving the setting on by accident. A line quietly sells 200 units past zero over a bank holiday, the supplier is on a part delivery, and there is no way to fill any of it.