The "fees" or "extra fees" or "customer fees" feature was meant to make
customers pay for the processing fee charged by the payment provider
they choose to make their payment. The fee was also displayed on the
payment form to deter customers and encourage them to choose another,
cheaper, payment provider.
In practice, it didn't hold up because:
1. the provider's API must allow sending the fee as a separate amount,
and PayPal was the only supported provider to do it;
2. charging extra fees is highly discouraged by providers, and forbidden
in Europe;
3. the final fee amount depends on the customer, country, payment
method, risk profile... rendering charging the actual fees amount
infeasible;
4. most of the time, only one payment provider is enabled at a time,
thus alienating customers who would have no other choice than paying
the fee;
task-3358581
closesodoo/odoo#132104
Related: odoo/documentation#5517
Related: odoo/upgrade#5053
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>