Welcome to the forum Newbie. Lets see if I understand this. A mail carrier hit your hummer with his side mirror and did $1,204 in damage?
You stopped your H2, got out, ran down the road behind him and had a pen/pencil/paper to write down a number while doing this?
He then returned, agreed to pay you "later", in cash ($1,204) based on what...estimate?
If all this is true it almost sounds funny except for your H2 damage and I doubt you'll find a "broken" mirror but you could order a new one from your dealer or GM Parts Direct.