more item trouble

[BuryAutumn]

is there a way to make it so you can buy the same item and have it stock up in one slot?

for example : you buy one potion and it fills up one slot. then you buy another and the amount in the first stock goes up by one.

i know there's a way to do it, i can't figure out how.

 

[Waaaaagh]

That would be called a stack system. I think we have one lying around here...