The first release of DateTime was in 2003. In 2008, I added a "DONATIONS" section to the docs, and around that time I started adding it to everything I released. It was basically just a link to web page with a PayPal button it, so not very professional.
Over the years since I started doing this, I'd estimate I've gotten somewhere between $1,000 and $5,000 (PayPal doesn't offer historical data far enough back to know for sure). On the one hand, that's a lot of "free" money. OTOH, this is software that has been used by absolutely massive companies, including Amazon (which I know for a fact used a lot of Perl in the early days, including things I worked on). So even $5,000 is an incredibly small fraction of the value this software generated.
Almost all of these donations were $100 or less and seemed to come from individuals. I can only recall one instance where someone asked for an invoice first, after which their company donated $500 (or maybe 500 euros), which was nice.
So this is just another data point.
And then I realized that this link has been broken since some time in 2018. Doh!
I'm fixing it and adding a "email me for an invoice" bit as well. It can't hurt.