Valid ID?

So, I visited markhams in Vosloorus on Monday, to purchase an item for Christmas. I then seen the 20% off on Thursdays for students, so instead, I decided why not take advantage of this offer? I took a closer look at the advert and it said please bring valid ID documents. (No where does it say you need to bring your student card) so, I went back yesterday, to get my 20% off, I took with me my proof of registration document, which clearly states I am registered at the university of Wi****ersrand for the current year, also, having all my details, with my ID no. and everything of that sort. I also took my end of year results and my drivers licence to confirm my ID NO. on my proof of registration. But instead I was told no, this will not work. The lady said I need a student card. Then she said come have a look, she took me to the poster and all it said was valid ID, so then she had nothing to say. I said have a look at my proof of registration, she did, she then asked for my drivers licence, I gave that too, and then she just said no we need your student card. Markhams, if that's what you want make it clear! Don't say valid ID. Is prof of registration and drivers licence not valid ID?