I have an idea/suggestion:
AirVPN could offer a matching contribution to the EFF. If you agree to give the EFF some of your money, AirVPN can agree to increase the contribution by (say) 50% up to some maximum. I think this is brilliant:
1) The idea is inherently sustainable. The contribution becomes proportional to AirVPN's revenue from users.
2) There is a nice sharing of social responsibility. If I think AirVPN should give money to the EFF, then I should be willing to give money to the EFF.
This could be implemented (for example) in the same page where you purchase your plan. You agree to give $X to AirVPN for so many days of service, and you have the option to add an additional $Y donation to the EFF and AirVPN will match that by 50%. Where $Y is some value between 0 and some multiple of X.
Btw, First post!!