charged a $3.50 federal Hazmat compliance fee
From one local site that does fills. So, that accounts for part of the $8.50 the other $5 might be a city thing.
The hazmat is either state or local flat fee. It's not as noticeable when you're doing 20# or higher. Prices also seem to vary widely. Still cheaper than those dumb prefilled tanks by the oz or L.
So, this site came up in my search for CO2 beyond just google options. I narrowed things down to a few places within the Loop / near the loop.
I had snapped a photo of a pricelist the last time I was hunting things down and IIRC the place was Big Tex on Long Dr and in my case 10# would have...