Hi Killua,
Is the form you looking for to interact with a processing company or one that you yourself will enter the info into your own offline/online system.
The link I saw based on the urls looks that should be fine. The only time you want a personal SSL certificate is when you do not want people to know your on shared hosting as well as not look as professional.
So the difference would be:
https://www.yourdomain.com/htmlfile.html
vs the free way:
https://server.hostingcompany.com/~u.../htmlfile.html
If you do have the Form email you the CC info, which somewhat negates the reason to use https:// in the first place unless you do secure email. Make sure to delete the email after you are done since you do not want to start storing the CC info in your email inbox.
If you want to accept credit cards over the net, PayPal has a reasonable method to do such that it will take their information and process it on their end. When dealing with credit cards though it is better to leave it up to third-party who deals with high-volume transactions to better secure the information. You do not want to have to explain to your customers that your website got hacked and their CC info was stolen.
Brian