It appears this international transaction fee is a fee charged by your bank not Backpage. Backpage shifted the processing of payments to the UK (outside the US). Thus making it an international transactions. Several banks charge an extra processing fee for using your debit/credit cards outside of the domestic US. I would discuss this fee with your bank or if you continue to use back page get a card that has more favorable terms for international transactions.
Hey I had this same problem I charge to dollars to post and then this random 12 cent charge shows up all the time . B.p. told me it's my bank charging me so I called the bank and they said it b.p. I emailed them and told them Noone authorized them to take this 12 cent it's not in the fine print and also girls I know who use a prepay card haven't gotten charge this. Imagine if they are skimming 12 cent from a million ppl worldwide that I'm sure use the site. Most ppl won't say anything bc I notice it months ago but never called until I notices 4 times in 1 day . Maybe we can get a lawsuit lol