Retailers paying them is what I understood from @Gatorguy, hopefully corrrectly. He will no doubt have a link if you'd like.
Returned items is a very good point, and as I understand it, a large portion were returned.
I'm not going back again to find it but the OP is welcome to look at International Accounting Standards, used by Samsung in reporting results. Essentially the IAS says sales are recognized in much the same way and using the same general rules as Apple does under US GAAP. Accordingly those requirements are usually met when the product/service is shipped.
As for returns I don't know that any of these guys, Apple included, generally report them.
Edited by Gatorguy - 11/22/13 at 11:21am