Just noticed this on my server. If you use the buy 10 button, it will give you 19 of the item you bought. Using buy 100 gives 199. Buying 1000 gives 1999.
I updated to v2.3.6 today and ran /update. I don't see anything in the config that would cause this.

Which currencies are you trying to use for your shops?

Im using the Economics plugin for currency

The latest update should resolve this problem.