A Tool to Calculate PayPal Fees

You can build a tool that help people calculate PayPal fees before they send out payments.

Here is an example:

https://www.salecalc.com/paypal

How to Make This Idea Better

You can add more countries and also enable users to do custom calculations.

You can also calculate how much you need to send a specific amount.

How to Monetize

You can monetize using ads. This is a no brainer or you can promote a financial or accounting tool as an affiliate and earn a commission every time someone signs up with them.