How to make a group of users on MODX who will be able to buy goods from a specific category with a fixed discount?

Question

CMS

I use MODX, minishop2. I need the ability to add users registered in the online store to a specific group (for example, TOP). Those users who are added to this group always buy goods from CERTAIN categories with a fixed 20% discount.

How can this be implemented?
Is there such a solution in the form of a plugin or something else? Thanks in advance