@Bunker - For vendors, I’d do two things to verify reliability:
(1) If the vendor is in the UK, see if they’re a member of BETA - the British Equestrian Trade Association.
(2) Check the helmet manufacturer’s website to find out if the vendor is an authorized reseller for that manufacturer.
Regarding customs fees in the U.S., you can take my word for it that a helmet should not be subject to any import fee, or you can do the research yourself by consulting the 2017 Harmonized Tariff Schedule. For helmets, you want to look at chapter 65 - page 9 - plus the section titled “General Notes,” which will help you understand what you’re looking at in chapter 65.
For anything other than helmets, you’d find the info in the same place, but you’d have to locate the appropriate chapter - of the 99 chapters in the manual! (No, I’m not kidding . . . .)