I'm sorry to hear that there has been confusion. Maybe, I can help point you in the right direction.
What do you consider cheap?
When you say whole domain what do you mean? Many SSL certificates will cover www./non-www.yourdomain.com. If you use a sub domain secure.yourdomain.com it would only cover that sub domain.
Being an ecommerce site are you looking for seals or warranties?
I personally use Comodo Positive SSL. It's cheap and gets the job done, all I really need from an SSL. Most internet users just like the little lock icon and the s added after http. A sense of security on the web could make or break a sale.
startssl.com do free 1-year certificates that cover a domain plus one subdomain (typically www but not necessarily). I use them and they work fine in all browsers I've tried.
Understand that StartSSL doesn't provide their free service for commercial services. And because they do very little to validate an identity their certificates would serve as a vanity SSL at best, which isn't very effective for an eCommerce site.
It's true that startssl free certificates "...provide modest assurances and are meant to secure personal web sites...". Specifically, they provide reasonable assurance that the user is communicating securely with the domain they expect, which is better than nothing and good enough for most browsers, but no reliable information about who actually controls that domain.
Verification of ownership obviously involves some checking and hence expense - startssl charge $60/year for an unlimited number of "verified" domains, which can be wildcard certificates (which I think the OP wanted). Whether that still qualifies as "cheap" depends on how many domains you want to secure, I guess.