From http://www.amazon.com/gp/help/customer/display.html?nodeId=2...:
FREE No-Rush Delivery (typical delivery one week after placing an order). Note: You may receive promotional credit for selecting No-Rush Shipping. Credit will be automatically applied to your Amazon.com account once the first package from a No-Rush Shipping-eligible order ships. You'll receive an e-mail as soon as the credit is available. You may select No-Rush Shipping as many times as it's offered to you, but you won't be eligible for the credit if you cancel your No-Rush Shipping order or return items from it.
And from their forums: "You get a $1 mp3 credit if you choose the slower shipping option."
One order may need to go by air if it's at a distant warehouse, but another may go ground because it's available close by.
I'd like it solely for the convenience--several times I have wanted to order something but knew I would not be in town when the delivery happened. I'd love to say "deliver it to me next Wednesday" and Amazon figures out the cheapest way that would work instead of having to remember to make my Prime order on Monday.
Imagine if I could have them consolidate those shipments; Get a weekly shipment for most items, or get it via normal prime when it's something I really need fast.