In the dark shop after you purchase an item you can also then quickly purchase another item at the same price say buying two items at 10k before the 5x kicks in, maybe lock purchases for a second until the 5x price of others activates.
amazing game really good that you guys are still updating the game even though it came out two years ago when most devs abandon games after a few months
Skillbooks do stack but each successful hit or heal that you get lowers the chance of doing it instantley. check the developer log for more info on that
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It's been fixed, just waiting on a good build to upload :)