I have never once seen a receipt for anything in a concierge lounge. The only transactions that may occur would be honor bar charges right? In all of these cases there's just a little form to fill out and leave with the attendant.

The only thing I've ever seen are tip jars which are fine but honestly I don't really feel the need to tip in most cases. It's a self-serve buffet and I always take my dishes up to the tray and leave them there. If there is some real notable service of some kind that's one thing, however this isn't really ever my experience.