I have shipped used and new. I find if you do not put a 'value' on the package, someone at duty in Belize will make up a number and it's often high so better to put on exactly what you think it is worth. And yes, do specify that it is 'used' if it is, but if you simply buy new sheets and just wash them and send them down, they will figure that out and make up a value based on new sheets. The process can be very arbitrary, I have heard all kinds of stories from a small percentage of duty paid to some outrageous percentage having to be paid. In my opinion, best to include photocopies of receipts when you can so they can't make up their own estimated value.