Woolworths online theft

Woolworths online advertises promotions, but when they don’t have stock of your preferred product, they charge full price for any substitution. In my case, I ordered a case of MCC. The promotion was -30% on any case of wine. When I arrived at the store for collection, they admitted to having no stock of the one I wanted and offered me a substitute (which had the same shelf price). When asked, the staff at the store said they did not have insight into charges. I had to reasonably assume that they would honour the promotion, since the cost to them was the same and the fault was after all theirs. When I received the emailed invoice, I discovered they charged me R810 instead of R567. They claim this is in their t&c’s. I have read their t&c’s and they are vague to say the least. Regardless of whether one interprets it this way (and I don't) this is an unfair and underhanded way to treat clients. I placed the order on 13 Dec and asked to collect on 17th. They have responded by email 3 times, telling me that I accepted the t&c's and they are refusing to refund me.