The scheme, outlined in an email sent to sellers earlier this month, lets Amazon deliver eligible products to customers in2,300 metro areaswithin two hours of an order being placed. Items sold through the service ‘have experienced 12% higher sales on average’ than those shipped through standard FBA, Amazon said in the email. ‘You pay only for units that actually ship through Sub Same Day, at the per-unit price you bid,’ Amazon told sellers. ‘Participation is optional, and you’re never charged more than the price per unit you set. ’ A new layer of cost on top of existing FBA fees People who advise sellers told Business Insider the bid system could increase what sellers pay to Amazon overall.