Changing price after purchase

I went to SS to utilise the free sunglasses deal. We were told any frame blue or pink and above would get us free police sunglasses. We proceeded to complete the transaction as stipulated. About 2 weeks after we took collection of the glasses and paid off what was told was owing I got a call saying we still owe money. Surprised I requested all signed invoices (which weren't given to me after the sale - ***** much?) When I received my wife's invoice I saw they charged an additional R600 because the frame she chose was cheaper than the police frame she chose. This R600 was conveniently added in pen after the fact with no signatures next to it (in other words not a legal invoice). If I knew it would have cost R600 more we would have never taken that frame. Clearly somebody slipped up and sold us something contrary to your rules. BUT we have a signed invoice with all the previous info. I never accepted the additional R600 charge so it cannot be binding. I might have been willing to play ball if someone phoned me with tears in their eyes after making a mistake but you chose to rather just give me over to collections and try your luck. I'm sorry, you cant change the price of a sold item