Amazon.com Help: Order with Free Shipping by Amazon
Add at least $25 of eligible items to your Shopping Cart. Proceed to
Amazon.com Help: Buy Add-on Items
Prime items shipped by other sellers do not apply towards the $25 minimum to purchase ...
Amazon.com Help: Availability Estimate Definitions
If you select a delivery slot after the Stock Until
time and try to add the item to your cart, you'll be asked to either
FREE Shipping by Amazon - Amazon.com
Add at least $25 of eligible
Amazon.com Help: Add to Upcoming Amazon Fresh Delivery
Add to Upcoming Amazon Fresh Delivery. In select regions, you can add items to an upcoming Amazon Fresh order if it hasn't already been prepared. To add ...
Add Items To Amazon Order Not Shipped For Free Shipping : Useful Links
Amazon's total purchase in order to qualify for free shipping is $49
whereas 58% of consumers add more items to cart to qualify for free shipping.
see here for detailed discussion ad nauseum
Because not everybody can afford $99/year.
When companies like Amazon first introduced free shipping several years ago,