Couldn't exchange sunglasses

I received a pair of sunglasses as a gift for my birthday on the 23rd September 2015. I went into Charles & Keith Centurion earlier today to exchange the sunglasses for another pair. I was even willing to pay the difference, if there was. I haven't used the sunglasses. I got to the store and was told I could not have them exchanged because there was no receipt nor did I have the plastic bag the sunglasses came in. I told the lady that it was a gift and chances are my friend does not have the said receipt or bag. I want to understand what your policy is in this regard? Am I unable to exchange the sunglasses (gift) because I don't have the plastic bag or receipt?