Nah, not going to allow them to put their advertising on my car like that. Must be a bad sign if they are that desperate to make it look like they are busy shifting cars on !
So what if they put their your stickers on your number plates. I don't think any garage has such an offer in place just to look busy. They are probably hoping to create goodwill with potential customers, maybe benefit from getting some warranty work through their workshop and the possibility of other repair work if they find anything wrong.
Go for it - you have nothing to loose.