put the follow in your page script:
%ITEMSTATE## := 1
The ## is the number of you item. If you go to your item list it'll say #1 or #2, etc in front of the item. Numbers generally are in the order you create the item in.
Examples:
%ITEMSTATE1 := 1
%ITEMSTATE4 := 1
%ITEMSTATE12 := 1
If you set it equal to 0, it removes the item from their inventory (assuming they have it, otherwise it does nothing).
You could also put it in the link script if of the link they click to get to that page if you want.