Checkers Tambotie Mall not selling me stuff from rain check they did

I went to Checkers Tambotie Mall with a rain check document that they did as they did not have the product in stock at the time. So when I went today, they had the item but they didn’t want to give me at the price written on the rain check. They say the rain check is old and upon asking where on the document does it say the rain check expires after how many days/weeks/months, they could not give me an answer. And non of them could tell me for how long a rain check is valid