How can I make shipping for some items free when used as upsell in my CA Store?
You can either just set the shipping to be $0.00 at the item level, or use the “free shipping if BIN” option at the item level. Upsell qualifies as a Buy It Now option in ChannelAdvisor. So you can set a normal shipping rate for that item, but say that it is free if used as an upsell by using that option.