I've done this crossing several times, at least once in this direction, but don't remember everything.
I have more than enough US dollars for the visa on arrival. But I only have $20 and $10 bills. (I also have some passport photos.)
Usually I have some $1 bills for just such border crossings because I know at several places there's fee, typically $1 for something or other, official or not.
I can't recall if the Lao-Chinese border is a place with the extra fee.