I can’t speak to this specific case, but speaking generally, debit and/or credit card transactions can incur flat fee, making small donations not cost effective, especially if the card is in a foreign currency.
A sufficiently small donation can actually cost the organisation money, so it’s very common to set a minimum donation amount. $5/£5 is likely safely north of that amount, but because fees can be variable, you do want to leave some padding in.
I can’t speak to this specific case, but speaking generally, debit and/or credit card transactions can incur flat fee, making small donations not cost effective, especially if the card is in a foreign currency.
A sufficiently small donation can actually cost the organisation money, so it’s very common to set a minimum donation amount. $5/£5 is likely safely north of that amount, but because fees can be variable, you do want to leave some padding in.