[FIX] payment_paypal: Paypal Acquirer fees

The variable fee (Var) and fixed fee (Fix) indicated by paypal must be subtracted from the transaction amount (Am).

So let's consider that Am = 100€, Var = 2.9% and Fix = 0.3€

Total fee = 100€ * (2.9/100) + 0.3€ = 3.20€

So in this case, the merchant will get 96.80€ and Paypal will get 3.20€

This computation can be verified with the simulator here:  https://salecalc.com/paypal?p=100&l=us&r=0&e=0&f=0&m=2&c=0.

So the idea here is to compute Am such that the merchant will get 100€ if he set his price to 100€

fees = amount_fees_included * percentage / 100 + fixed
fees = (amount + fees) * percentage / 100 + fixed
fees = (amount * percentage / 100 + fixed) / (1 - percentage / 100)

if the merchant needs to keep 100€, Am will be equal to 103.30€.
That way, paypal takes 103.30 * 2.9% + 0.30 = 3.30 and the merchant takes 103.30 - 3.30 = 100

opw:2369557

X-original-commit: 6850e638dfa652ab3fe8065ea09e23cb07648d7a
Co-authored-by: AdrienHorgnies <aho@odoo.com>

addons/payment_paypal/models/payment.py

@@ -81,7 +81,7 @@ class AcquirerPaypal(models.Model):
         else:
             percentage = self.fees_int_var
             fixed = self.fees_int_fixed
-        fees = (percentage / 100.0 * amount) + fixed / (1 - percentage / 100.0)
+        fees = (percentage / 100.0 * amount + fixed) / (1 - percentage / 100.0)
         return fees
 
     def paypal_form_generate_values(self, values):