Swiped transactions for American Express: 3 percent plus $0.15 per transaction.

Keyed-in transactions for MasterCard, Visa, and Discover: 2.3 percent plus $0.15 per transaction.

Keyed-in transactions for American Express: 3.5 percent plus $0.15 per transaction.

Groupon will also offer its payments system to merchants who don't use its coupons, though at a higher rate. For non-Groupon merchants, swiped transactions will be charged at a rate of 2.2 percent (3 percent for American Express) plus $0.15 percent per transaction.

Compared to other payments systems, Groupon's service -- even for non-Groupon merchants -- is a pretty good deal. However, comparing mobile payments processing systems is difficult.