I would suggest an SSL Certificate just for the simple reason that it makes your clients feel safe.

That being said, with your current setup; don't quote me on this, but I think that since your going though paypal which is a secured site and your customers are not giving you money directly then I don't think you are required to have an SSL Certificate.

In regards of what I have said above, I would highly suggest that you have one anyways.

Best Regards,