Great concept and good for the Steemit community.
As @Simplysara points out, the costs at this time are more than using Amazon direct; and, as you accurately reply, that is due to the fees associated with converting Steem/SBD to BTC, and then BTC to fiat. You note that that is about $10 at current pricing.
Alas, the dogmatic challenge of all cryptocurrencies during these infancy years; the dreaded processing fees. Kind of like the ole "shipping and handling" fees that would get you following your purchase of a late night informercial product during the 90's. It would be of a product you just had to have, was not available in stores, and if you act now you'll get two for the price of one. S&H fees or not, you bought the product.
Flash forward to now (post internet), and most shipping and handling fees are nonexistent or minimal.
Yes, currently all cryptocurrencies face too high of fees, but with time and continued acceptance and usage, these fees will diminish, if not become todays "free shipping and handling" from crypto exchanges just to get and keep your business.
@Steemit is a great platform to generate cryptocurrency for enlightening posts and concepts. I'm all in.
'Up-posts' to @paywithsteem for thinking outside the box, identifying a need, and creating a channel to fill that need.
Post on @Steemit, earn Steem, shop at @paywithsteem: shopping with free money. What could be better or smarter.