If it were at all possible I'd love to have a way to tie the backpack size to economics. For instance if they want one row they pay X, and again for 2nd row and 3rd or however many you want to implement on a server. 

Something like this can be achieved by using custom shop plugins. For example, create a listing in GUI shop which costs Economics and purchases a permission.

Mmm interesting, I'll have to look into that more. Do you have any idea if they can have dependencies as well? For instance if to buy the 2nd row you had to purchase the first row, and so on throughout the levels. 

Ideally shop plugins should support that feature, but I don't know if that feature exists today.