at a guess i would say they are an 'add on' purchase as opposed to a specific purchase, so the postage is a standard rate across the board for most items, i guess that most people would buy say a hoodie and think yeah i'll have some stickers aswel at which point the postage is sort of irrelevant but as you say buying them on their own comes across expensive.
why not buy the hoodie? get the stickers aswel, and a mug, possibly even a beanie, it is almost winter, is your head cold? the postage will seem quite cheap then!
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