Too much to resist...
In 1984 I was passing through the UK on the way to emigrate to Canada with my Canadian wife. We had to go through the last processes through the Canadian High Commission in London, and while we waited we managed to get a cheap time share in Malaga, Spain and were to fly out of Gatwick, but at this point we were in Manchester.
Now at the same time the Animal Liberation Front had threatened to spike Mars Bars with rat poison to make a point, and of course that impacted sales. So they were offering all sorts of deals to make people buy them...
Returning to our trip, we presented ourselves at the bus kiosk and asked to purchase the tickets. The ticket agent said, "Where are your Mars Bars?" As we were obviously totally confused by this the fellow sighed heavily and explained that if we each presented the MRP codes from 11 Mars Bars we could get the tickets at 60% off - a significant saving. He told us to go to the supermarket across the road and come back with the codes.
We went across the road and got to the checkout, presenting our 22 Mars Bars, at which point the checkout operator asked "Oh yes, going to London are you?" When we got back to the bus counter we offered the bars to the ticket clerk. He opened a large file drawer to reveal it full of Mars Bars. He said "I just want the bar codes, not the bars!" So armed with our tickets we headed off to Spain.
It turned out that Mars Bars were not available there and we managed to off-load our surplus to expats who were grateful, despite the threat of poisoning!