This is exactly what you should do. Your users will understand -- that's why so many have supported you on the googlegroup. Charge $3/mo and you won't hear any complaints (and if you do, you can point to your googlegroup).

This is by far your easiest and clearest path to monetization, at least that I can see. And it seems you're long overdue for monetization.

Thanks hrjet, that is a great idea. But again I will have to struggle about the payment gateways. Thanks to HN guys for helping me out.