[abeyer]

Donations from companies are notoriously difficult to push through for any but the smallest/newest companies. Any more established ones tend to have rules and bureaucracy built up around the process of making donations (if they do at all), and individuals within usually have little direct influence.

Paying license fees for tools or software, on the other hand, is commonplace enough that small amounts are usually at a manager's discretion with little oversight. Offering to send an invoice for a 'license' that doesn't offer any additional benefits may be a way to simplify things for companies like this that want to pay to support the open source they use.

[tajen]

I second this: Open-source projects, please send invoices so companies can bury them in their accounting. Even postgresql.org let me down with a "the only invoice is the CC receipt from paypal", which triggered a specific discussion with my accountant about the legal limits of donations (France).

[regularfry]

It can also be straightforward to get individuals in to give a talk, or training, on what a bit of software can do. I've done that in the past where paying directly for the software or service itself isn't an option for whatever reason.

[haser_au]

I third this.

An invoice is enormously easier than making a donation. Most large scale organisations give even low level managers an authority limit (up to X amount) where they don't need to engage the full Contracts and Procurement process. If you're asking for less than $1k per annum, you'll find a lot of large scale organisations will happily pay. And the people using these tools are often very supportive of open source, and want to support these projects. It's in their interests; you maintain the library, they get the benefits of having bugs fixed.