Google Express is an interesting marketplace where you sell your products on Google. The conversion rates and CTRs are significantly better, especially on Mobile, and it’s very compelling to sell there.
The feeds are not so simple though, because the Google Express feeds require multiple steps with a product feed, local inventory product feed, and order and tracking feeds sent via the Google Content Shopping APIs.
The latter Google Express feed API involve Acknowledging the Order, Submitting tracking, and updating shipping through the APIs.
If you can get on Google Express, it’s well worth the effort as you can now be found on a great channel through web and mobile apps.