One little quirk is that Groupon does NOT collect sales tax, so we're required to collect tax at the time of redemption.

We need our POS system to handle a payment of type "Groupon" for $X and then a second payment for $Y.

So - for example - if a product is $100 and tax is 5% (total due: $105), we need the total due to show as $105 (naturally), then apply the Groupon payment of $100 and show a balance due of $5, to be paid by credit card or cash.

Last I checked Square register doesn't support this, Shopkeep doesn't support this, and a few others I checked don't either. It's not rocket science - just the ability to accept 2 or more payments for each sale (Groupon + cash or Groupon + credit card) and have the balances get calculated correctly.