I'd take him out for a very nice lunch... after lunch I'd lay out for him what happened, show him what your exact costs were, and see if he would have it in his heart to at least make sure your costs were covered. Let him know what your typical margin needs to be, and that it is an industry standard type of mark -up. See if he would be willing to negotiate a lesser margin. Offer him a couple year window of him being able to get stuff through you at cost? $2700 bucks is a big chunk for anybody. If he wants to be a hard-ass, he really wouldn't have to pay - legally. Hopefully you did a nice install for him, and his guilt might get him to contribute to your miscalculation!

If not, like Carl said... a learning experience. Maybe you could also volunteer that he wouldn't have to pay it all at once --- could spread it out over time?

I forgot to add in the price of a projector, in a non-line item priced theater. I just gave him a grand total figure, which ended up a few thousand below my costs. It was similar in that I screwed up when I was doing it in Excel, but was manually tallying up the items. I was sick...

I was so embarrassed, I waited a while to tell the client(s). They met me half way, and I broke even. It sucked!!! I'm pretty A.D.D. and now I always have a couple people in the office look over my numbers before I send them out to a customer... I've also made the mistake of going off the suppliers' price list, and charged for (1) each instead of pairs of speakers!!! DOHHHH!!!
Or put down the dealer cost instead of reselling price :(

Good luck man! Just another reminder for me to triple check.

A few years a go when I worked for a pretty nice retail operation, a customer called up our bookeeper and told her: "Hey! I love my 55 inch Elite TV, but I've had it for three years now, and you guys haven't billed me yet"!!! ---It wasn't me, just another knuckle-head in the store that buried his financing paperwork under a stack of trade magazines in the office!!! (I think they were CE-Pro), and totally forgot about it!