Can companys change what theyv agreed to even if its not in the contract?

i have a store card and insurance cover. i signed up for the insurance 2 years ago, the contract said that if i kept the recipts and took them back to the store every year if the item id bought was on sale id get the difference back on my card. now the the store has stopped offering the insurence ( i think the didnt realise the amount of people who would keep their recipts) the insurance company are now saying that if i dont send the recipts back within the first month of buying the item then i wont get my money back. can they do this even if i still have my contract which says otherwise?