Fwiw, here's the how the math works out for a Prime membership.
@ 8%, you need to order ~$1100/yr to make your money back. Thats for stuff you would normally have to pay to ship. Case amounts ship free anyway. They say if you dont save at least the $95 dollars over the course of the year they refund you whatever the shortfall is, iow if you saved $85, they send you a check (or credit your account, Im not sure which) for $10.
If you order a good amount of ammo over the course of the year, or if you typically order in amounts that require you to pay for shipping it can be a good deal. Depends on how much youre ordering and how often.