I just traded in my iPhone 7 for a XS and they didn't charge me $15 for "New SIM card fee". I used the same sim card. and there was a sim card in the box as well.
However, they charged me $20 Upgrade Fee.
It feels like this is an American thing, we don’t get charged anything extra in the UK. What the hell does an “upgrade fee” mean anyway? Are you upgrading earlier than you’re supposed to?!