I've never seen one of these yet where the business didn't pay for the customers 'free' financing.
There is no such thing as free money, somebody will pay for it. It's usually you. It's all a game. You offer them no interest for X amount of days, the finance company charges you $500 for the pleasure of financing your customer. You are expected to jack up the price of the job $500. If the customer doesn't pay it off in full within the term it converts to a short-term loan at a high rate and usually all the interest that was waived is now acrued and due and put on top of the amount financed.
I'd instead maybe ask them if they have a credit card. Have them call the 800 number on the back and have the credit card issue them some of those checks. They can write you the check and they can pay back the credit card. Then if they want no interest they can transfer the balance to a new card and get no interest for awhile.
I'm tired already just thinking about all that mess.