Most of the time when modding/fixing things that are not game consoles then most people would rather not burn me (similar logic to don't piss off the person making your food). If I was doing general repairs for the public (I reckon it is better than working in an office but not by much) then it would be money before you get your device back -- if it is test case for a repair then play that how you will.
If you do want to do payments after the thing is sent back then you either charge enough to wear the occasional dine and dash (or ask for a downpayment if you really want), learn how small claims court works, do payment escrow services (credit cards will tend to call it sometrhing like pre authorisation), have some kind of system with reputation that you can then cause trouble for (I doubt you will be able to ruin credit scores here but it is not impossible) and so on. Generally though for small things like 3ds modding (case, nand and such is small, if you are installing video capture kids or something then that is a different matter) it is not worth trying to sort payment after the customer has the device back.