This is becoming a rather strange habit,, but Sean is spot on.
Your a buisness, not a charity, friend, community worker doing rival businesses a favour. If people can't get what they want from tesco's they go to asda - that is tesco's fault for not supplying the correct service, not asda.
You've been approached so the work is fair game.
No point in undercutting as you end up slaving away for little reward and end resenting the job.
Darran