You can donate by Paypal to firstname.lastname@example.org.
You could also send a check to:
Help With Math
185 View Court
Aptos, CA 95003
If neither of those options is convenient, send email to contacthelpwithmath "at" gmail.com.
You can also send email to that address to tell us why you felt like donating.
The author of MathXpert, Michael Beeson, is a retired professor of mathematics and computer science.
He spent seven man-years over a 12-year period writing MathXpert.
What is MathXpert worth to you?