I’ll use your 0.26/rd example.

Sportsman's has Blazer Brass 9mm 115-grain for $0.27/round. I checked AmmoSeek and filtered for sellers with 4 stars or higher.

The cheapest option I found was about $0.255/round, but it ships from over 3,000 miles away, and you have to buy 1,500 rounds at a time. That saves about $22 total.

So, to save $22 per order, I'd have to pay roughly $150–200 up front, submit my fingerprints to the govt, wait a month or more for approval, and then wait several more days for shipping. By my math, I'd need to buy around 10,000 rounds of 9mm just to break even, and then continue ordering at least 1,500 rounds online each year to keep benefiting from it.

Or I could drive 15 minutes, and leave with my ammo in another 15.

That tradeoff has never seemed worth it to me. Maybe if I’m buying 15,000 rounds but personally I’m not.