i am new to NZB in general and newer to SABnzbd, i have used nzbget for few weeks now and have been using RSS ever since. Hopefully i am able to help you out.
As per wiki:
Then, new feed items in subsequent RSS reads will download automatically (if the feed is enabled). Also make sure that your SABnzbd is not paused like thisTo prevent the download of a (potentially) large backlog, the first batch of NZBs will not automatically download. These lines are marked with an asterisk *. If you want these matched feed items in this first batch to download, click the Force Download button.

To check if your feed is set up -> you need to do is goto RSS tab and click on "read feed" next to "edit"

Once you have that open, pay attention on proccessing, select the one that makes sense to you and fits your purpose.

See wiki for more info on each options

EDIT:
Also to add, since you mentioned that this is cart rss feed, which means once rss is feeded by SABnzbd, they will be removed from cart (if feed url contains &del=1) and will no longer be available for next feed.

