I just spoke to them on their website's livechat/Olark -- they're very responsive and helpful. Also the service is free so no reason not to play with it.

What are those features?

We've been selling a ton of product via Groupon and need a POS system to handle the redemptions of those Groupons.

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.

Isn't that the same as any coupon? Why not set up a "Groupon Coupon" that applies a $100 discount. It should end up with the sales tax at the end like you need.

I don't think customers are expecting that. I wouldn't.