Repaying a loan and processing fees

Question

Here's the (hypothetical, of course) scenario: Reuven borrows $20 from Shimon. The easiest way for Reuven to pay back the money would be to send it through one of those new-fangled fancy electronic methods (for example, PayPal). Shimon is willing to accept payment in that manner.

The problem is that PayPal takes a percentage of the transfer ( 2.9% from the sender if it's a "transfer to a friend, 2.9% from the receiver if it's a "business transaction").

So the question boils down to this: Who is responsible to pay the processing fee when paying back a loan via a third party? For the sake of the question, let's assume that we're dealing with a default case where nothing was agreed upon in advance (although if this touches upon issues of Ribbis, it may not matter whether the parties agreed to it or not).

Answer 1

Your question was asked here.

The question was

As a Company we accept payments from customers made via their credit cards. We are charge by the bank for each transaction a fee of 0.67% of the transaction. 1. Can this fee be on charged to the customer?

The answer was

Adding on the charge that you incur from the Credit Card company is permissible, and poses no problem of Ribbis.

There's more detail in the linked article.

Moreover IMHO Shimon surely will not loan money to Reuven again if he has to accept only 97.1% of his money back.