Leave from SJU or STT into ATL(DL) or CLT(AA/US) and no Customs and Border Patrol or TSA in arrival airports. We did both Jun and July respectively in 2014.

Buy duty free in both, continue as carry-on on connecting flight! as no TSA check on USA landslide.

Not the case in CUN. did that dec 2013 into ATL. You have to clear USA CPB and TSA arriving ATL. Could have duty free booze bought after CUN security , but only if ending in ATL. If connecting, you would have to place in checked baggage, couldn't be carry- as more than 3oz 100ml rule.