The gap is longer than the term
Large retailers pay small suppliers on net-60 to net-90, some on net-120, depending on category. The clock starts when they receive the goods, not when you ship them, so a truck that leaves in January and lands in mid-February starts a ninety-day count that pays in May. Add the retailer's payment run and the deductions and chargebacks that come off the remittance, and a supplier is commonly out of pocket for a third of a year on every order.
Suppliers describe it accurately: the retailer is running on their cash. There is no leverage to change the terms and no real choice about taking the account. There is a choice about how to fund it.
Fund the first order, then fund the cycle
The first order is a bridge: a working capital loan or a cash advance on your existing deposits, sized to the production cost of the order and funded in one to three business days, with the purchase order and the retailer's terms attached. An underwriter reading a dated PO from a national retailer next to your statements is reading a receivable with a very reliable payer at the end of it, and offers reflect that.
The second and every order after is a cycle, and a cycle wants a business line of credit: drawn when you pay your supplier, repaid when the retailer remits, costing interest only on the days it is drawn. It is harder to qualify for and slower to set up; apply for it after the first remittance lands and the statements show the account is real.
Shrink what you borrow
Your own supplier's terms are the cheapest financing in the chain; net-30 on your raw materials is thirty days of the gap you do not fund. Most large retailers run an early-payment or supply-chain finance program that pays inside a couple of weeks for a discount — ask the buyer for the program's terms and compare its cost against the line. And price the wait into the unit cost from the first negotiation, because the retailer's terms are a cost of goods, not a surprise.
The trap
A big-box account that becomes most of your revenue is a customer that decides your payroll date, and the deductions on the remittance — compliance chargebacks, shortages, promotional allowances — are real money that the PO did not mention. Keep the account under half of revenue if you can, read the vendor manual before the first shipment, and fund the cycle with a product that can absorb a remittance that comes in short.