Just my opinion, but I would ask the "client" what their expectations are for the price, if the response is, "oh, I don't know how about $20 each or something" I would respectfully decline.
As far as I know bragging rights never paid a bill or allowed anyone to purchase food or gas or equipment. I mean even if the guy is your friend, your friend should help you out and pay you a reasonable amount for your work, right?
I would politely tell the store that you would like to be in charge of the printing of your images so that you can make sure the final images look as good as possible, and are mounted to a very high degree of standards (with UV protection etc.) and mark up the cost of the job by 50%, so if one print costs $300, add $150 to that and charge the client $450 per print (just an example, printing costs could be much higher).
That way the store won't use the images more than they are agreeing to and you can get paid a fair amount for your work and time.