Practical Fees & Surcharges

Excellent app! Simple to set up and use, lots of options to suit any sort of custom fee you can think of, as well as a bunch of presets.
The UI is really nice and modern. It also includes a sandbox for testing rules before they go live and dashboards for seeing the impact of your fees.
Simonas has also been really helpful in making a few tweaks and fixes.
Really great, with a generous free tier too.