I was frustrated & puzzled hugely as well, until I realized there's multiple fulfillment centres in my metropolis, and not all will have all the items. So while

I was frustrated & puzzled hugely as well, until I realized there's multiple fulfillment centres in my metropolis, and not all will have all the items. So while I ordered 5 items that I feel are logically related (e.g. "Guitar Music stuff" or "Computer stuff"), in reality they might come from multiple centres on multiple trucks. Amazon will usually mention an option "send using smallest number of packages" I think when they are from same centre but with potentially different shipping times. (and that's for "Sold and Shipped by Amazon" stuff; I haven't a foggiest how things which are Sold by XYZ but Fulfilled by Amazon; vs Sold & Shipped by ABC are done. I noticed in such situations they'll come with wildly different couriers as well; there's one minor company that absolutely positively does not actually deliver or accurately deliver packages to my house, but I haven't figured out the pattern/trigger and my complaints have gone to deaf ears)