I regret I can't help you on the JMail, as I'm not familiar with it. But somebody will

By "link to", he meant any page that points to your final form where they put in sensitive info must start with https://
As for transferring data from the server securely, you can encrypy email and have a key to decrypt, but unless you spend money, you would have to use something like PGP with a public key; that is a potential risk. Or, you can secure download (see below).
I will give you the same advice I give my web customers.
Let somebody handle the dirtywork. Lunar's support team will help you get set up or set it up for you (for a fair price) and make sure all is in place to safely transact.
Or you can go to
2checkout.com, give them their $50 setup fee, and let them take care of it. They provide secure server, transaction, and a merchant account. They'll deposit to a bank account or mail you a check every 15 days.
There's nothing wrong with doing it all yourself, of course. The folks at Lunar will go out of their way to help you get and stay set up. But I'm sure you realize there is a lot to secure online ordering, and it could save you 'tuition' to pay someone who does it regularly and will handle all the 'dirtywork'. If you have a merchant account and you want to manually enter sales into your card machine, then I would suggest possibly a secure (SSH) file transfer at least twice a day, with possibly PGP encryption.