Alright, I have donationcraft plugin so people can buy donor ranks I just not sure to make Rank Upgrades so players don't have to pay full price for when they buy another donor rank, so they only pay the difference from the rank they are currently! THIS IS URGENT I NEED TO KNOW VERY VERY VERY BADLY!
I believe this can be achieved by enabling 'Cumulate item purchases inside this category. Purchasing an item counts towards others.' within the shop category settings.
