It does make me wonder though - if tax setting "Payment method additional fee is taxable", what should the result be for a negative fee? You can't charge tax on a negative. It would seem that the negative fee acts like a discount on the subtotal - should, therefore, the Tax for the order be calculated on the "discounted" subtotal?
Today I discover which the negative fee was only a workaround over the lack of PaymentMethodAdditionalFee (AdditionalFee, AdditionalFeePercentage) on the PagSeguro payment plugin. Then I try to place a negative fee on payment method to compensate this, I do but this is not the correct. In this way I need increase the price of the product variant to compensate and place a negative fee in the other payment methods. I have fixed partially including the options(Additional Fee, Additional Fee Use percentage ) and save in configuration view of the plugin and the configurations are saved in Configuration\Settings\All Settings but I receive a error message: Object reference not set to a instance of an object
Ok I finally get the fixes to plugin and now all works fine: install, uninstall, AdditionalFee, AdditionalFeePercentage, save settings, order sumary, ExtraAmount, working with pt-BR culture Language, object reference. If all works without issues I upload a update today.