I've thought about this since I run some services. If people get repeat customers, you should be able to get repeat vouches. I don't think I need to make paragraphs about this since its a simple concept.
You can simple edit your previous vouch and add on the price of your second/multiple deals. I think the reason for a simple vouch per member is to prevent vouch spam or manipulation of some sort. Just simple edit your previous vouches if you have a same customer, that's what I've done, I've had several several deals, but only like 6 vouches, because I have customer repeats.