Add the ability for customers to add gratuity tips to invoice

My customers frequently leave gratuity for services performed and often ask if to have it added to the invoice. What I would like is for a gratuity option to pop up when the customer swipes their credit card just like it does anytime you pay for other services like a salon or restaurant. I have not locked in on which credit card processor I'm going to use with RepairDesk, but this option would definitely make me choose one over another. Thanks!