I use AMZ Refund every 2-3 months and it always gets back significant money that I can't find myself via account audits. They only charge 8% vs the 25% that the other main player does.
Also, this service is one of the only ones that is within Amazon's TOS, as they don't send in 100% automated refund requests (it's semi-automated, as they find the inventory/refund/missing item mess-ups, and then gives you a template to submit the tickets to Amazon, which makes it TOS-ok).
All in all, I'll keep using these guys. Recommended.