I don't think it is possible to create a certificate via FTP, but there is a solutionto your client's unwillingness to pay for one. How about a free certificate?
Let’s Encryptis a free, automated, and open Certificate Authority recognized world-wide.You may create the certificate via theSSL For Free website.
Another well-known furnisher of free SSL certificates isCloudflare Free SSL/TLS.See this pagefor further instructions on setting up SSL with Cloudflare.
Free certificates can also be created via theComodo SSL Free Trial,limited to one issuance per domain.