Question about fiat vs crypto value

Hi, I still consider myself fairly new to crypto. I hope this isn’t a stupid question, but it’s been eating me for a while now. My question is… I live in the U.S.A, when purchasing goods from a foreign website, do I use the crypto value OR the fiat value? I’m kind of confused. I know that some foreign sites conveniently tell you what BTC value to enter, however, will this be the equivalent of (say) UK value or U.S. value? Should I always check the exchange value first?

Also, do I have to pay additional taxes for making purchases overseas??

Thanks…

Damn! I waited so long for help I don’t need it anymore… That sucks… I definitely understand not to pay for any service here.