Seriously though, Fastly's donation of their CDN service is generous and eases the burden on the PSF, but if push came to shove they could definitely afford the bandwidth. In 2018 they had a net income of half a million.
There's absolutely no way the PSF could afford PyPI's bandwidth out of pocket. Last I checked our "bill" from Fastly would be close to $1.5M/month.
Also given that PyPI is critical infrastructure for millions of people and software projects, anything cheaper would not really cut it.
For example CDN77, would start you off at 0.016/gb if you have more than 100TB per month, without any negotiation.
If you have 300TB per day, you surely can negotiate sub-cent pricing somewhere.
No, the costs are manageable.
I think the charitable/community projects setup for developers, need to act like non-profits in other sectors and actively seek out donations.
If PyPI didn't have sponsor providing bandwidth, I'd guess it would implement some form of rate limiting and encourage people to mirror/cache packages much more to reduce load. I don't think it would die completely, but it would be less convenient and still cost the PSF a fair bit of money.
You are right that this donation does not show up in our tax filings because they provide it to any OSS project.
"If you don't support us, we might accidentally forget to audit our packages and feed malware into your build pipelines. It'd truly be a shame..."