JoeN:
(though in my case all come from U.S. companies, just sometimes from their factories overseas.)
If you are buying from a us company and you are based in the US you do not pay customs. The US company payed the customs and calculated those in into the price.
If you buy from a non us based company someone else (that is you) will need to take these things into account.
Another option: You are not located in the us ]
Best regards
Jantje