First, the invoice
Before any funding: call the customer's accounts payable, not your contact. Ask whether the invoice is approved, when the next check run is, and whether anything is holding it. A surprising share of 45-day-late invoices are sitting in someone's approval queue with nobody chasing. That call costs nothing and is faster than any funder.
Then check your own contract for late fees and the right to stop work. You may not want to use either. It helps to know you can.
The bridge: working capital, sized to the gap, repaid when the check lands
A short-term working capital loan or a cash advance, underwritten on your last three to six months of deposits rather than on this invoice. Decision in about a day, money in one to three business days.
Size it to the shortfall, not to the invoice. If the invoice is $40,000 and the gap between now and your next receipts is $18,000 of payroll and rent, borrow $18,000. Everything above the gap is cost with no purpose.
Ask for weekly payments and ask what early repayment saves. If the customer pays in two weeks, you want a product where two weeks of borrowing costs two weeks of money. A fixed-factor advance charges the same whether you repay in two weeks or six months, which makes a term loan the better shape here if you can get one.
The actual problem is concentration
A customer who is a third or more of your revenue can decide your payroll date, and one that pays late has just done so. A funder will notice the same thing in your statements: a large single deposit that arrives irregularly reads as risk, and it prices that way.
The fixes are the boring ones. Deposits on new work. Progress billing on long jobs instead of one invoice at the end. A card-on-file or ACH authorization for the customers who will give one. Net-15 for the ones who will accept it. None of them are exciting and every one of them shrinks the gap that put you here.
Before the next one
A business line of credit is the product built for exactly this: drawn when a receivable is late, repaid when it clears, costing nothing in between. It is the hardest of the four to qualify for and slowest to set up, which is why the time to apply is a month when the statements look good, not the week a customer goes quiet.