In the U.K. when I have bought items from the USA and the taxes were not taken care of by the shipper (this does sometimes happen with larger suppliers eg Dick Blick) then the postman rang my doorbell, showed me my package and asked for the payment if I wanted to receive it and I paid him on the spot. This was in a very rural area. In towns you may get a letter from the post office telling you to go in and pay the duty and collect your item. If you don’t pay, they don’t release the item, they send it back to the sender. Not everything attracts customs duties, though. Art supplies do. I’m not sure about art work.
I don’t know what happens elsewhere - only the U.K.