I just ordered my Media Link HD and ran into the same problem as you.
Tried it four times via the website and each time "We're sorry, Something happened while processing your order. Please try again."
So I called the 'Order by phone' number, this number for me -> 1 866 866-7509
I explained that I am not a Sprint customer, but I was trying to purchase the accessory and got an error.
The gentleman went through and got all my information, but at the recap of the order he told me the shipping was $12.50.
I asked if there were other shipping options, as the website told me 6.50 for shipping. He told me this was the only option and recapped the order again. I was about to stop him, when he said "So with the accessory, the local tax, and shipping the total will be $21.67."
I was confused and thought he forget the shipping in the math, but I didn't care and just let it go through.
Well, I just got the confirmation email, I paid a total of $21.67. Somehow I got free shipping.
Checked the bank to confirm. Guess I got lucky somehow!
Thanks for the tip OP! I've been wanting one of these since I got my One X!