Lie to get assistance

I am highly disappointed in the service that I received from Jet Elsies River. I purchased an item on my account but unfortunately it did not fit. When I tried to return the item (with tags attached and receipt at hand) I was refused as I could not find my card - refused by shop assistant, shop manager and customer services number printed on back of receipt. I indicated that I simply wanted a bigger size but still I was informed that without my card they could not process the exchange. I was informed that it is a policy of edgars group that they cannot exchange items bought on account without the account card however on inspection of the receipt it became clear that this is not contained in the terms and conditions of exchange printed at the back of the receipt. Customers are therefore expected to be aware of policies that are not explained or communicated to them on the receipt. Am I expected to lie in order to exchange an item that I have proof that I paid for - should I hide my receipt and say it was a gift to get an exchange (a discretion often exercised by management) in order to accommodate a customer? Why am I not entitled to a gift card or basic exchange if I am not satisfied with my original purchase regardless of being an account holder. Should they not be more accommodating to me as a loyal card holder? How can a company refuse to exchange an item for a bigger size if I can't find my card. Clearly my money is good enough to accept as payment but they won't allow my to at least get the right size