Agree with HappilyInsane and mime, among others. I don't think there's anything wrong with saying, "Jamie, thanks for thinking of me the other day, but actually I prefer to pick the parts myself. I've got my system down and having someone else do it just slows me down." No need to mention Mary's name at all, if it's not really something to do with her personally--you don't want anyone to help with this.
Perhaps you could also think of something else Jamie could do to help you, that you could tack on. "...But you know what you could do, is have someone fill out Form X for me, which would save me a good 20 minutes each day." That might make him feel better, and also get a benefit for you.