Tee Sony Ordered a shirt, needed a bigger size. Return policy says I can exchange it but they refuse to Charlotte Internet

 I ordered a printed teeshirt in a medium but when the person I bought it for tried it on it was a bit too small. The return/exchange policy stated that if it did not fit I could return it within 30 days as long as it was unworn and was returned with the original packaging. Now they are telling me since it was "custom" I can't exchange it and that it was my fault for ordering the incorrect size. It is not custom, it was chosen from a large selection of preconceived prints. Usually a medium is the correct size, but it depends on the shirt. I am very unhappy with the customer service from this company and they have pretty much ruined what was supposed to be a birthday gift.