i still consider ecwid to be ta great option and they've got even better recently. the only downside is that the free plan is limited to 10 products I think.
However, they have great support and there's an affiliates program etc.
I've asked the NING support about this and the answer is NING won't make changes to specific lines of code for individual networks. And they suggest me to move to another shopping cart. I'm still working on it try to find solutions. Or I need to find another shopping cart.