Returns. The sale is the easy direction. A return has to cross every system: authorisation, label, goods-in, a decision about the condition of the item, stock back on sale or quarantined, and a refund or credit. For fashion retailers, with return rates of 25 to 40 percent, that's where the hours go.
Stock across several locations. Once you've got more than one warehouse, a 3PL and a marketplace, somebody has to decide what's really available and where an order should ship from. That's business logic, and it belongs in one place.
Money. Shopify payouts, refunds, gift cards and VAT all have to land correctly in your accounts. If the totals are out by pennies, someone will spend Friday afternoon finding out why.
Busy days. Integrations that look fine on a quiet Tuesday can fall over in a sale. Rate limits, queues and retries need designing in from the start, so a spike slows things down rather than losing orders.