I'm guessing the reason why you thought they were from the USA is because the have a .US domain name. Unfortunately the .US domain does not require the registering party to be located in the USA, unlike many other country-specific domains, which is a shame, IMHO, because it gives the visitor a false sense as to who they are dealing with in the case of an online store.
Everyone and anyone can register a .US domain. The only rule the .US domain name has is that you cannot use privacy on the whois info of .US domains. As you can see, their domain whois info lists the registrant as a China-based individual/organization.
https://whois.domaintools.com/smokstore.us
Next time you make a vape purchase and you need the vendor to be from the USA, check their contact page and about us page for an address. Also, if a vendor takes Paypal they are not from the USA because USA vape vendors are not allowed to use Paypal.
Also, the shipping options are a dead giveaway, too. If you see shipping options like "e-packet", "HK Post", "Airmail", "EMS", or weights measured in kg, they are not a US-based vendor. Long delivery times for the cheaper shipping options are also a dead giveaway.
But as JL said, don't worry too much. If you used a credit card or paypal, you are pretty much protected should things not work out as it should. My guess is everything will be OK. They are not a fly-by-night company. I have not ordered from them but I have seen many threads on forums of other people that have.