I picked up Borderlands this week and am really enjoying it. I'm playing the Siren and use primarily
Repeater Pistol and
A little while back I purchased an Ammunition SDU from a vending machine. The SDU increased my SMG ammo up to I think 720.
Now whenever I visit vending machines that I come across, I still have the option to purchase an SDU for the same Ammunition. I figure you can just continually upgrade a certain ammo type and max it out (at an increasing cost), but the tooltip seems to be telling me that purchasing the SDU will increase my ammo to the capacity that I already have (720) and cost me $6000.
Is the tooltip misleading or is there actually only one SDU for each ammunition and they keep it in the shop for the sake of having it populated?