+1 on the duty free. I have yet to find any bargains (though I am not much of a shopper).

Here is a general warning about buying duty free liquids (i.e. booze) at your originating airport and the transferring through PTY on US-bound flights. (Ex: MDE-PTY-IAD and buying duty free at MDE):

With the US-required extra security at the gate, you cannot bring liquids on board. If you buy liquids at your originating airport, you won't be allowed to bring them on with you at PTY unless you somehow have access to your checked luggage. You can buy duty-free at PTY, because they will give you the liquids on the jetway, just like in the USA.

This would apply to any airport that has secondary screening at the gate, not just PTY.