Not “boni”.
That is all.
Nearly two years of silence and you sign in to say that. 
I think you’ll find it’s actually “bonusi”. 
Come on, everybody knows bonus is both singular and plural form 
Well, I only recently got back into Grim Dawn again.
About 2 years ago I told someone this in one of the topics, he argued vehemently that boni was correct.
Now that I’m back and reading the forums again, I see “boni” everywhere, more than ever!
Seeing you on the Steam forum frequently too!
Yes, I hang out there as well. 
Whoever told you that is wrong, bonuses is correct for multiples. Boni’s probably become the slang word for it. Or else people just can’t spell.
I’ve to point out, in “German” “Boni” is one of the plural forms of “Bonus”, so that could be also the reason for it…
bonus is latin so boni is the proper plural form (in latin), but bonuses ended up being used in english and is kind of considered “right” at this point, probably even more so than boni.
but I also rage every time I hear someone quote Caesar ‘veni, vidi, vici’ wrong - it’s not italian 
the downside of being forced to learn latin at school…
I remember hearing a word everyone is saying wrong thinking it comes from latin, when it really is from ancient greek and plural has a different ending. Can’t remember the word though 