If I were you I'd go with a private SSL. Since your hosting company offers a Shared one that means that your secure area URLs will change. For example
Instead of
https://www.yourdomain.com
it will be:
https://secure.yourhost.com/www.yourdomain.com
or
https://111.111.111.11/www.yourdomain.com
(depends on your host)
You get the jist. Even though your site is secure your customers don't know that and it will look like your redirecting them to a different site. This could increase your dropped carts/orders.
If you check out:
https://www.securepaynet.net/gdshop/ssl/ssl.asp
The standard SSL is a cost effective choice with a simple whois validation process (meaning you'll only need to click on a link in an email sent to the whois admin contacts email). The link above is what the good zen people have to offer. It's comparable to sites like godaddy with the same pricing. Network Solutions also offers ssls and their seal is real nice but it's a little more expensive and has a more rigorous validation process.