I have a similar situation.
I have my regular customers and my wholesalers.
I have a group called Wholesalers, and they currently get their discount, however some wholesalers have their own shippers, and account number so for those we do NOT charge shipping.
I also have a second group of wholesalers that get a lesser percent off, but still have their own shipper.
So if I understand correctly I can make a new zone, and use the code from the first example but change the line of code from
if (!IS_ADMIN_FLAG && $group_query->fields['customers_group_pricing'] != 'wholesalers' or 'wholesalers35') {
But I am afraid that adding a the zones will effect my payment set up, am I able to add more than one zone to my payment types. Is there anything else?