Most forums using this software are set to only return searches for 4 characters or more. You can try sim*, but you'll get any word which contains that string.

Anyway, if you're talkng about authorize.net, the AIM is more secure, I guess. You want an SSL certifucate either way, as you're collecting personal info from youir customers. You don't get the cert from authorize.net, though. You can buy any cert and your host will install it (usually). Though they shouldn't be used for Zencart hosting, GoDaddy has cheap SSL certs.