Is it permissible to use a prepaid credit card for the purpose of running da'wah advertisements on Facebook, knowing that it charges a fixed issuance fee and an 8.5% fee on each electronic transaction (6% transaction commission and 2.5% foreign currency provision commission)?

Dealing with a debit card is not a matter of a loan, and the rulings on usury do not apply to it. The additional charges added to the purchase value or cash withdrawal are not considered usury. The bank is permitted to pay foreign currency on behalf of the cardholder, then collect its due from the cardholder's account in local currency, or exchange it first; because constructive possession is achieved, and there is no harm in the bank setting the exchange rate. The bank has the right to charge fees for issuing the card and other fees for providing services. It is permissible for these fees to be a percentage of the amount transacted.