At the moment FOSSBilling doesn't have any functionality to sell SSL certificates I'm afraid.
Not sure when/if that would be added, you're the first person to have asked for it and I think these days most people expect free auto-generated certificates or will directly source ones from trusted certificate authorities if they are needing that for something like PCI compliance..
And yes you can donate to FOSSBilling!
There's a donation dropdown on the website which will let you select between donating via GitHub or OpenCollective.