I think "kind of" is being used as a modal marker here, similar to how "like" is used. In particular, this could have been said as "which honestly like upset me

I think "kind of" is being used as a modal marker here, similar to how "like" is used. In particular, this could have been said as "which honestly like upset me", but because of some negative backlash against excessive use of "like" as a marker, people have switched to using other words. This particular use of the word is trying to "soften the blow" of the discomfort that the listener (in this case, reader) would feel at the un-modified phrase. So if you just said "which honestly upset me a lot" that might seem like an extreme reaction for just a price increase of some service (it's not as if someone is dying), so the "kind of" is added to signal to the listener/reader that the speaker acknowledges that this is perhaps too-strong language for the situation.