A couple of club members in my local club have gorren lots shipped and had to pay the delivery company a customs COD payment.
I went to the US Tarrifs calculator page and found that used stamps do incurr a customs fee. It's based on value. so a $25 value package will incurr $34 dollars of customs fees.
On
ebay, everything from the UK says buyer will pay customs upon deliver in order to receive the item.