I set up a server with the plotme plugin: http://dev.bukkit.org/bukkit-plugins/plotme/

I want to enable players to donate and get reward with 1 more plot. So if they currently have 1 then they buy the package and get 2, if they have 3 and buy they now have 4, etc....
But I do not know how to do this with the way plotme permissions are set up. Any suggestions?

plotme.limit.X

The Bukkit page said to "Replace X by a number to limit the number of plots a user can claim/auto or buy. plotme.limit.* for unlimited plots. If the user doesn't have permission, the default is 1 plot. If the user has multiple permissions due to inheritance, highest number is kept"