I bought this game before the guide book was available. Is there a way to buy the guidebook without rebuying the entire game over again?
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According to this forum post, the only way is to do a refund and re-purchase at the higher amount. Seems like itch is planning on changing this in future, but who knows when that will happen. It looks like you'll have to contact the itch support to do this. Sorry for the inconvenience :(