Customer discounts

No, it is not possible without manual intervention. Custom fields cannot be linked into any calculation for the simple reason that the program has no way of knowing what you might put in there.

Without knowing what exactly you are trying to do, I can only guess at suggestions. But you might be able to simplify things by defining non-inventory items. Suppose you have three levels of membership. Definie three different non-inventory items, such as (1) Basic, (2) Deluxe, and (3) Golden Trowel. Mark them as “This item can be sold.” Define different prices. Now you can invoice someone easily by choosing their level of membership as a line item. If membership rates rise, edit the non-inventory item. See the Guide:

Once you have the first sales invoice created for a member, set up a recurring sales invoice. Now you will be reminded when their membership is due and don’t even have to worry about what their level is. See this Guide: