If you live in a different US state to backcountry.com you don't have to pay sales tax though right?
Yes and no.
Yes they might not charge tax.
But you're still expected to declare it on your tax return and pay sales tax in your own state (which may be zero, but probably not).
Used to be that they could get away with it, but since the likes of Amazon et.al. and flat rate shipping made it possible to really take the piss they've cracked down, now the big retailers have agreements in each state to collect the tax up front.