Well, each certificate is good for one domain - part of what you're buying is verification that your site is the legitimate site for that domain (the other part is the encrypted communication provided by SSL).
If you think your users will be accepting, you can always set up a single SSL site.
Another possibility is buying a wildcard cert, but those are pretty expensive.
Finally, you can use a third-party payment portal allowing you to run an ecommerce site without SSL. All payment information is handled by a third party like PayPal or Google Checkout.
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