Forcing customers to spend more

I visited the Engen convenience shop located on Higginnson highway at 09:20 am where I purchased 2 bottles of water. When I presented the items for payment I was advised that when paying by card at the store I would have to spend R20 or more. I asked if this was Engen policy and was assured it was which is odd considering I've not had to do this at any other Engen convinience store. Some clarity on the issue would be appreciated. And if that is the case I will in future take my business elsewhere.