tax / calculation rule of shipping product

[Grisu74]

Hello, I have a tax / calculation rule of shipping product and enabled the option "overwrite final" in price product, my question is why when I use custom fields variant cart you change the price of the discount and the final price ? I think it is a bug because in doing so the discount increases more and more and does not increase the price, so we close the store before you start! , D; D thanks in advance for your answers

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