I can't speak for the system operators, but I'm sure that donations are always appreciated!

You can donate via Paypal using the "Make a Donation" button on the landing page: http://sdf.org/. You can alternatively mail cash directly to the following address:

SDF Public Access UNIX System
Post Office Box 17355
Seattle WA 98127 USA

Donation is actually one of the ways one can "validate" an SDF unix account, which enables additional shell commands. When donating via Paypal, simply write "Validate [SDF Unix username]" into the message area to get validated. For mail-in donations, you would instead write your SDF username onto the upper-left corner of the envelope.

Crypto donation is apparently also available by request. You can contact membership@sdf.org for wallet info.