The inherent problem with voucher sites like Groupon is that you pay up front, so if a merchant screws you or won't accept a coupon, you're very probably stuck dealing with the voucher site. Enter Voucher Complaints.
Put together by Harvard Biz School assistant professor Ben Edelman, it generates complaint letters based on your situation—and this is the killer part—based on your location, because laws vary from state to state. For instance, did you know you might be able to redeem an expired coupon in Massachusetts for the full face value? Check out this hardcore noise:
I have a concern about the expiration date on my voucher. I tried to use the voucher after the listed expiration date, and the merchant told me I could redeem only a lower value, not the full face value shown on the voucher. Under Massachusetts law, your voucher is a gift certificate. (Specifically, the law states: 255.1: "A gift certificate shall include... any other medium that evidences the giving of consideration in exchange for the right to redeem the certificate, electronic card or other medium for goods, food, services... of at least an equal value" (http://bit.ly/mf4SSH MGL §255.1).) According to Massachusetts law, a gift certificate may not lose value due to dormancy (MGL §266.75D). In particular, see Massachusetts 266.75D: "Whoever sells or offers to sell a gift certificate, as defined in section 1 of chapter 255D, which imposes dormancy fees, latency fees, administrative fees, periodic fees, service fees or other fees that have the effect of reducing the total value amount for which the holder may redeem such gift certificate, shall be punished by a fine of not more than $300 per violation" (http://bit.ly/lw8kV0 MGL §266.75D). Thus, the reduction in value is not permissible.