Questions for OP:

• Where are you getting your exchange rate (USD to BTC)?
• Do you have different rates for the $10, $20, and $50 increments?

Comments:

• I was hoping I could pay with credit card points, but it appears not.
• Looks like I can pay with Amazon gift cards, though.
• The $50 denomination shows $0.00 for shipping/handling.  I think that goes with Amazon's free shipping on orders > $25.