Even if they restrict the donations to only a few "eligible products" (which doesn't seem to be the case [1]), 0.5% for a retailer is an incredible amount of money.
Wondering if they're compensating this move with tax rebates. Amazon already operates under razor thin margins; they can't afford to take another 0.5% from the bottom line.
[1] From http://smile.amazon.com/gp/charity/pd.html/ref=smi_se_saas_p... Purchases Eligible for Donations Tens of millions of products on AmazonSmile are eligible for donations. You will see eligible products marked “Eligible for AmazonSmile donation” on their product detail pages. Recurring Subscribe-and-Save purchases and subscription renewals are not currently eligible.