"Get'em into the market and half your battle's over; impulse shopping does the rest" says a writer in a grocery-trade magazine.
So Mr/Ms Manager, get your [bonded] security firm to release an apparently unmarked "Lucky Trolley" into the trolley ranks as if it were a card going into a pack and then
thoroughly shuffled so even the security staff themselves couldn't find it.
But a sensor "planted" at a lot-drawn checkout by security staff would blow its top when the Lucky Trolley arrived.
The purchaser would of course, with appropriate fanfare, get the contents of the trolley free.
Disguising the "rebound" type trigger on the trolley would require some thought.
And what would be the medium? Radioactivity? Ultrasound?
About cheating, Murphy's Law says it will happen, even if the prize be small; however the same person or family winning several timrs would arouse suspicion.
Other bugs? More than a dirty dog has fless, ... but R & D is usually tax-deductible