It is a good idea! But when I add a tax to the fiscal position, I need to specify a "tax source" and a "replacement tax". If I want to add a new tax, without removing anything, how can I do? Because the field "tax source" is required.

You map, existing tax as source to new tax. New tax = old + included sub tax = re_charge.

It is not a solution. I need to maintain the current taxes because the credit card re_charge is applied AFTER of the other taxes. For example: my default sale tax is 21%. If a sell a $10 product, then the price with taxes is $12.1 and with 10% of charge is $13.31 if I do the new tax = 0.21 + 0.1 = 0.31 then $10 * 0.31 is $13.1 != $13.31 ... do you understand?

Tax constructions has more options and may include first tax in base amount for next. See https://doc.odoo.com/6.1/book/3/3_9/tax_structure/

Yes, I know that. I need to specify a default sale tax of 21% and set "Include in base amount" and sequence = 0. But then, what tax need I to replace in the definition of the fiscal position "credit card"? This is my problem...

I solved the problem! I create a fiscal position and in the "Taxes assignation" I set: Sale default tax 21% replace with -> Sale default tax 21% AND IN ANOTHER LINE sale default tax 21% replace with -> Re charge credit card 10%...