Marketpress wrong procedure

Hi all.

I use the plugin marketpress, and would like to bring attention to a procedure of the plugin.
Once you have calculated the total, will pass to the function for calculating the fees, then the function of discount.
But I think this is wrong! For example, there are some products that have different taxes, such as food. And how do purchases that should have different taxes?

Another example: If I add the field categories to the discounters, such as the discount only applied to selected categories?

I say this because I have modified the plugin to select the categories which apply the discount. and tax different for different products (Something similar to shipping cost)

What do you think about?

Best regards