Cash payment requested by employee.

Good afternoon,

I have a question about the cash payment I was asked to make at o2 shop back in January - Right after paying 121E with my card as first payment for my S9+ mobile phone contract, I was asked to pay another 50E - specifically requested to be payed in cash. This was not a one-time connection fee as those were billed to me on all of my mobile numbers. What was this payment for?