Here's how it looks like from my side as a seller. 1. Buyer places order, order goes into pending for anywhere from minutes to days 2. While order is pending,

Here's how it looks like from my side as a seller. 1. Buyer places order, order goes into pending for anywhere from minutes to days 2. While order is pending, units of inventory are moved to another fulfillment center 3. Once the units arrive, are received and checked-in, the order goes live 4. The customer receives their order two days later I suspect this happens 1. for items less often purchased which have little historical data to rely on, and 2. items which have been freshly received and haven't yet been distributed across the US. I am a very small time Amazon 3p seller, so I might be able to answer some questions you have about Amazon or FBA.