The same thing happened to me when I bought my Evo. They demanded 2 accessories or no sale. Said it was an order from the regional manager (San Diego). I called customer service from the store, and they said it was not acceptable to make that demand, but they were unable to order the store to drop it. It was a Sprint company store, not a third party store.
I went ahead and bought the accessories, and got the last Evo in the store that they were about to give to someone else. I called customer service after I got home, and they agreed to reimburse me the cost of the accessories I bought. I just had to scan and email them the receipt. I did that, and after the 30 days passed, they credited my account. So, I ended up getting a cover and case for free.