I confess... I saw this thread before and totally blew right over it.
I thought, oh gee - here we go again.
Yes, there probably have been a 100 threads about this and those who have gone through them come away with a bit of cognitive dissonance I guess.
SBI was set up as a do-it-yourself guide to build informational websites. Everyone was riding high on AdSense and the like. Simple. Build a big site with the most cherry keywords in the niche and profit off people who are surfing.
That is not working so well these days.
This is just not set up to handle the myriad of complications that come with setting up a serious e-commerce website. There are credit card processing issues, SSL certificates, shopping carts, checkouts, and on and on and on. Trying to get that tied into SBI is like banging your head on the wall (I have the bruises to prove it!
I don't know, but I suspect that the new Biz express (whatever) launch was done precisely to address issues like this. I admit I don't know, but it would seem that they would have a plug-in or two since they are doing WP sites now. Might be worth taking a look.
Sorry I couldn't give you something more meaty to get your teeth into, but if you snoop around in here, you will find A TON of similar threads. Type in store or cart into Find It and you will see what I mean.
Good Luck man!