I bank with First Direct. I just googled 'non-sterling transaction fee first direct', and found this text:
You will normally need to enter your PIN at checkouts. For non-Sterling (foreign currency) transactions we will charge a fee of 2.75% of the Sterling amount of the transaction. This fee will be shown as a separate line on your statement as a 'Non-Sterling Transaction Fee'.
http://www1.firstdirect.com/1/2/travel-and-international/tra...
And in my experience there's no minimum on this fee, i.e. the effective percentage isn't any higher for small amounts.
I agree with other people's comments about using Revolut and Transferwise cards for $ purchases in general. But if the commenter to whom I replied is spending $0.50 per month in USD, then the non-sterling transaction fee should be ~2 pence per month. So not worth the extra (albeit minor) hassle of maintaining another card.