"A bottle of rum, cigars and cheap souvenirs" - memorize it and say the same thing every time. You never know when you're going to get a new/overzealous customs agent and it sounds a lot more honest than being unprepared or saying "nothing".

Our friends buy waaaaay too much jewelry every year and they declared it once not knowing any better. After wasting a bunch of time and scrambling to find $39 or whatever like the OP (they would only take cash) one of the agents whispered to them saying not to make a declaration like that ever again.