HTC Sensation :: Volume Rocker Broke - No Satisfaction From T-mobile Or It?

Had my HTC Sensation break today.Only had it 3 months so it's still under warranty. The volume rocker fell off. Called T-mobile. They said it was a known issue and would be covered but they said I had to call HTC. So I did. THEY said I had to mail back the BACK of the phone because it was an ACCESSORY! This was not a separate purchase. It was the back that came with the phone! The regular back cover. And after mailing the back, back to HTC they would mail me a replacement. Now if this is a known issue, why isn't T-mobile dealing with it as a customer service? I shouldn't have to mail it back to HTC. It is already an inconvenience to have to drive to the T-mobile store. Or HTC should cover postage. When Dell has to replce a part, they send you a prepaid return label. It is the last HTC phone I will buy and may be the last time I renew with T-mobile.