Is it safe to use Web Invoice 2.1.0 without SSL, (if only interfacing with it using Paypay)?
I have not used that plugin but I would assume (I hope) that any payment interface is handled by the payment processor. That means that any credit card information is PCI complaint and not stored on your server or have anything to do with WordPress.
In addition, the secure communication between your server and the payment processor must be done with SSL and a valid recognized certificate. Be aware that there are liability laws and insurance that deal with this depending on where you live, etc, at the very least have a user policy your clients are aware of. Personally I would never use anything like this to store confidential or private info.