Welcome to the forums! Good question,
You’d need to read the policy of whatever store your purchasing from.
Some it seem will charge sales tax, if you’re in the state that they’re selling in. Some require you to pay a fee to cover the PayPal cut that gets taken if you choose to do goods and services over friends and family.
Ultimately morphmarket isn’t going to be charging any fees and taxes, you just need to be sure you fully understand the sellers terms.
Same for shipping as well, some sellers include shipping, some charge actual shipping costs, and some charge a flat rate to everyone.