We thought we’d give you a break with the hard puzzles and let you have this one, but oh well. The answer you’re looking for is ‘2534’.
There are four colours on the stamp of the postcard, those are blue,green, purple and red (in that order). On the other side there are 2 blue potions, 5 green potions, 3 purple potions and 4 red ones. All together this makes for the code ‘2534’.