Comfort Zone Bed Shop Athlone, Cape Town

We have purchased a double bed that was guaranteed to carry the weight of up to 80kg,a 60kg person has slept on the bed and in a week's time the mattress was flattening out. We had contacted the store and spoke to the sales lady who then told us that she will replace the bed with another one exactly the same. Why would I want to have the same bed that would last only a week. They refused to acknowledge my concerns and asked us to pay more if we require a better bed. It has been almost 3 weeks and nothing has been done.