You only let Amazon simplify your life in that manner because you don't care about potential savings elsewhere.

Many of us , myself included, after filling up a prime-enabled cart then go to walmart.com or some other such e-commerce retailer and then do exactly the same process you described still to check the price against Amazon.

Many times I go with the other retailer due to cost savings.

If Amazon prices things and incorporates shipping costs in such ways that people know they must double check them against other e-commerce vendors, where does the convenience and time savings come in?

What i'm saying : You're taking for granted that your convenience comes at a cost, in most cases.