But one get one free it's a *******

I bought a pair of glasses at Spec Savers Noordvaal Madiba st Pretoria in Feb 2020 during their special buy one get one free. Everything was done and processed and I received my 2 pairs. 2 months ago I received an email saying I am owing R200. At first I thought it was a **** and I ignored. The emails kept on coming until I approached the shop. I was told it's the difference between the prices of a free sunglasses pair and the normal pair. I feel ********** because they never explained this to me on consultation or on collection. I was not going to buy the same glasses if I was told on day of purchase. This is absolutely unfair. Not enough information was provided to me that if my sunglass frame is more expensive than the normal glasses I will pay the difference . Which I do not have. I am frustrated and this is putting a financial burden on me.