That's the problem, the 3rd party can charge whatever they like for a handling fee. However, they should return a breakdown to the merchant, so the form could then look like this:
Purchasing: L$12,190
Subtotal: 44.95 GBP
Handling fee: 4.01 GBP
Total: 48.96 GBP
It's simply down to what details the 3rd party sends back to the merchant and nine times out of ten it's a simple handshake 😒