I just noticed something. I ordered two items I wasn't in a super hurry for and Amazon told me I would get them on March 4. This morning they showed as "pending" still. I ordered another item that I needed ASAP and this evening I got a message saying all of them were shipping in the same box.
In short I think if you choose slow shipping to get $1 or whatnot on digital goods you can just place an order later for one item with prime shipping and get it all shipped in two days anyhow with the benefit.
It might be worth testing.