Charges Multichoice can't seem to explain

The other day I went to Multichoice to make a payment of K1700. I had written the cheque beforehand and upon reaching the till the conversation went as follows
Teller: The amount today is K1722
Me: Can I pay the K22 in cash?
Teller: Then you will have to pay K522
Me: On top of the K1700?
Teller: Yes
Me: Why?
Teller: Well you have to ask over there (pointing to the customer service attendants)
Me: So if I pay the whole amount in cash what will I pay?
Teller: K1722
Me: So where is this extra K500 coming from?
Teller: Ah, well you will have to ask over there
Me: I am just surprised where this K500 is coming from!
Teller: ……………. i.e. silence

When I asked the Multichoice guy (the tills are contracted out to First Merchant Bank) he was understandably disappointed that the teller failed to explain himself. In his explanation he implied that FMB collects commissions on every transaction and that on the K1700 bouquet their commission is relatively low. It was a vague explanation but I assumed my K1700 cheque and K22 cash would have been treated as two separate transactions both attracting a commission. The K500 was to cover the commission for the cash.

It’s just interesting and quite sad at the same time that I was expected to pay so heavily, a third of the bill, for wanting to top up with cash. What is even sadder is the fact that no one was willing or able or both to explain where these charges were coming from.