Amazon is forever tweaking this or that policy with respect to shipping fees. They're now moving to a subscription fee model with their Prime Pantry setup.
It used to be if you were a Prime member then you could use "Prime Pantry" as a filter to shop for a range of shelf-stable / nonperishable grocery and household items, fill up a box that maxes out at 45 pounds with certain dimension restrictions as well (they use an appropriate size box for smaller orders) and they'd ship it for a fixed $6 fee. Ground shipping, not guaranteed two-day. So it would be 1-4 days.
Now Amazon is rolling out a subscription model... where Prime members can sign up for a separate membership in Prime Pantry, incurring a monthly fee of $5, and then if you load up on at least $40 during a "Prime Pantry" expedition, the shipping is free... still ground shipping, so 1-4 days. And same as before, items not marked "Prime Pantry" that you stick in a cart while trying to load up a Prime Pantry box will get shipped separately, not in the pantry shipment.
"It's complicated",,, yeah. If you limit your choices to pantry items, but you don't spend the $40 pantry box minimum, even if you opt in for the monthly membership of $5, then a pantry box will now ship for $8.
Yep, that's the punchline. One box ships for 8 bucks now, not 6, unless you opt for a $5 per month sub (and make the $40 min buy). I guess if you expected to shop Prime Pantry a few times a month it could seem reasonable. The caveat is that Prime Pantry stuff is not generic, it's brand names so you're already possibly paying a little more for stuff if you sometimes buy store brands at a supermarket. Clearly this gig is about convenience, not your budget.
Walmart on the other hand does keep it pretty simple and offers free shipping for $35 of groceries not marked "In Store Purchase Only". All you need is an online account and plastic to pay for the order. It's also your option to pick it up curbside at a specified local store free of extra charge.
If you're not inclined to use either of these services very often then the Walmart option can seem attractive. I resorted to that a couple times during the spate of nor'easters we had, when I couldn't even dig my car out of the snow no matter that the guy had finally managed to plow the driveways. They delivered via FedEx to my back porch in 2 days both times.