At my local market the other day I wanted to buy some kitchen sponges that were priced at 10 for $1.00. They were displayed in a hulking big bag, maybe 5 or 600 of them chunked down there.
I checked my wallet and found only 80c in change apart from a $50 note. So I thought I might get 8 sponges for my coinage, yes??
Well, no apparently. I could not buy 8 sponges for 80c. They’re 10 for a dollar or nothing.
Except they would sell me five for 50c. But not 8 for 80c. 
When queried on the logic and mercantile sense of their decision, I was met with the dumbest of looks you can imagine. It’s like I was trying to rip them off with my transaction…no matter how well I explained my coin-poverty and willingness to pay 10c per sponge, they were not convinced.
Yes folks, it’s people like this who vote in our elections. 
This gave me a chuckle, and I could see it happening in any number of shops near me. Not only do they vote, but they breed and raise up another generation to take their place…
They may not be eligible to vote.
Also, buying 5 for 50 cents or 10 for $1.00 means they only have to count in 5’s when they’re doing stocktake.
If you buy 8 for 80 cents, they’re two short or three over.
See.
That’s how to run an economy. You weren’t in the Federal Treasurer’s seat were you?
It does make sense actually. Compare it to a 24 pack of your favorite soft drink for $4.99. They’re not going to let you buy only 20 units of the pack at .21 cents a can for $4.20. They want to sell a pack of 24, not 20.
I also couldn’t tell from your post, but the offer of 5 for $0.50, was it a five pack, or individually listed? If listed individually, then I can see dumbness of it. But if listed at a package of 5 and 10, I wouldn’t expect them to break them up and sell individually.
It may be that the EPOS system is set up with only one or two product codes for the item - e.g. 5 sponges=50c and 10 sponges=$1. You may have been asking for something that it was impossible for the cashier to ring up correctly.
Also, if the system was sufficiently relaxed as to sell you 8 sponges for 80c, they’d pretty much have to sell one sponge for 10c, which they may not want to do, given that most people not given the choice to buy one for 10c would shrug and accept buying five for 50c