If you do a quick search on daniweb or google, you will find a lot of negative comments about 2CO. This is particular to seller selling e-products. I do not integrate 2CO to my sites because of this. If 2CO is your only choice, they do provide their own shopping cart.
Well, I have integrated 2CO on 3 of my websites. It is true that they suck. Their support is horrible. My scripts broke many times because they made changes and did not bother informing their customers.