I say "Hi I'm your evaluator from XXXXX and you just won!" I smile brightly while saying this, then say something like "Good job, you upsold well!" I found from experience that if you sound like you really know what you're talking about they are less likely to disbelieve you and get nasty about things. I did one two days ago, handed the girl the certificate and she went flying for the manager who gave me the eye. I was very relaxed and matter of fact, showed her from a distance the rest of my paperwork (so she couldn't grab it like one guy did) and I matter-of-factly told her she had a good employee. Everyone got all happy and they left me alone to measure and write those blasted prices, which are different in every darn one I've done so far.
If it isn't successful, I say something like "I'm your evaluator from XXXX" and then hand them the other certificate saying something like "This is to let you know I was here." I then just get the info I need, say something like "I need to look at a few things, take some measurements and I'll keep out of your way." Again you say it matter of fact, friendly and polite and move around like you really are from corporate and know what you're doing. If the employee eyes me up while I'm measuring the purse rail and whatever, I throw them off the scent by measuring a couple things that don't need measuring, nod my head approvingly and keep moving. By the time I'm done I've circled the place and end up by the door where I slip away and escape.
Her Serene Majesty, Cettie - Goat Queen of Zoltar, Sublime Empress of Her Caprine Domain