TLDR: They assume you won't ask for a receipt and tack on extra charges to make back the loss on the discount given. I fueled up, got a coffee and here's my receipt. I didn't get a slurpee, and they only told me about being overcharged when I asked for a receipt.
For once in my life, I have some info that might actually be useful for the general public. This might be common knowledge or super obvious to some but.. it wasn't to me until today.
Today, I fueled up at a 7-Eleven with a fuel price lock. ULP (91) 47.12 L @ $1.639/L, this came out to be $77.23.
I had a fuel lock of $1.21/L so I received a discount of $19.88.
I bought the $1 coffee as well. So the total should now be $58.35.
Clerk said $61.35 and me not thinking much of it, just paid for it.
Then she asked if I wanted a receipt, usually I don't but this time I did. She had a surprised look on her face and hesitated and said "Oh sorry I accidentally overcharged you $3.. do you mind if I refund you with cash".. again not thinking much of it, I just said sure why not. Thinking it was an honest mistake. Then I got my receipt.
I have a feeling that they do this to a lot of customers since most people, like me, probably don't bother with a receipt. Also the expression on the clerk's face when I said I wanted a receipt was also kind of suspicious. Could be an honest mistake though, but I'm sure if I hadn't asked for a receipt, they wouldn't stop me walking out of the store. It's not much money, and I'd still be saving money at the end of the day, but it is still dodgy nonetheless if intentional.
Anyways! Always ask for a receipt when fueling up with the price lock!