Hope this helps:
I have purchased older or vintage H bags from Japanese resellers, shipped to CA through Japan Post and USPS here. Usually I had to go to my local PO to pay the duty before USPS released the items. I’ve never done the exact calculation but I think it was around 8% of declared value which was my purchase price. None of the resellers I buy from has ever suggested under declaring.
I’ve not experienced any hassles nor been disappointed by the items but I don’t do ultra high $ purchases with international online resellers.
I’ve also purchased from Collector Square in Paris, one purchase was shipped to me through UPS and they do collect the duty. I don’t remember the exact % but it was under 10%. The bag was less than $4000.
if you want to look at more US resellers:
Rebag accepts returns albeit a very short window and also has a buyback program like
Fashionphile.