Ok... so this is a thread that is nearly a year old and honestly, it hasn't moved.
Now, I don't mind getting my hands dirty and helping with this but right now it would be a case of the blind leading the blind so is there anyone who can assist with figuring out how things have changed and what the correct settings should now be?
Once I have an understanding of what's what I'd be happy to dedicate a little time to writing up some documentation and examples.
@websnail Maybe try leave a message in the forum post: https://forum.piwik.org/t/adwords-tracking-final-url/15703 where other users will maybe have some idea or guidance.
Coming back to this 18+ months on and having reviewed the minimal threads on this it's apparent that there's very little understanding of how Google Shopping differs from Google Adwords.
Every forum and GitHub thread on this topic has resulted in an assumption that Google Adwords is the same and if you check back you'll find that there's numerous "Why don't you try posting here" suggestions which serve to send end-users like me on a wild goose chase that takes us back to square one.
I don't want to belabour the point but Google Shopping is NOT the same as Google Adwords. While the process of setting up advertising via Google Shopping is handled within the Adwords admin system, the variables, links, etc... are decidedly not related.
In the interests of getting this resolved, I'd be happy to provide access to a Google Shopping campaign, (as well as links to the piwik instance for the associated site) so dev's can actually see what's going on and get to grips with the process.
If the net result is that Google Shopping isn't trackable as a campaign then that would at least clarify functionality, but right now there's a whole lot of assumption and not a lot of knowledge so I'd really like to plug that gap.