As to my knowledge, a double spend needs quite some preparation and timing. So to do it on a POS-situation, would be rather hard. You need to insert two transactions on two distant places in the network.

For a coffee or a beer, that would not be worth the trouble. For a car or watch, it might be.

I do not know about the guarantees of Bitpay, but for small amounts, it should not be an issue. If you do sell luxurious watches or cars, do double check. For these amounts waiting for 2 to even 6 confirmations would not be too much of a pain.