Coupl'a thoughts.
Many items there are sold by AMZ but also by individual vendors. Some ship their own items, others send them to AMZ for them to ship (Fulfilled By Amazon). Many do both.
If you look at Widget 123, the price shown on the product page could be any one of those three options. Usually you can click a link to see the list of all vendors selling it and their prices including shipping. Sometimes you can sort those lists in different ways. Sometimes not.
So, if you see Merchant Pat''s Particulars in the sell box the first time you look at the product page and click Add To Cart, that's the merchant and price you'll be buying. If you don't check out that day, but come back a few days later, you may see a different merchant and price. Why? Maybe Pat's sold out and now The Wonderful World of Idjits has the item at the price closest to what you originally selected. Or there could be other reasons.
Now here's the kicker .. FBA stuff can be included as Prime while items shipped by the merchant cannot. FBA adds extra fees to the merchant's monthly bill. So, in some cases you may see the merchant selling the shipped by them version for less than the Prime item since they won't have to pay those extra FBA fees. You may see Widget 456 selling for $15.00 and 'free shipping' under Prime, but $10.00 + $3.50 shipping if shipped direct from the merchant.
Last thought .... their 'Add-On Items'? You can't buy them alone, you can only add them on to a qualifying order. They claim they do that due to the cost of shipping small items. However, on more than one occasion when I placed an order with an Add-On, that single small item shipped by itself from a completely different state than the body of the order. And it came in a box far too big for the item.
