WHAT IS THE DIFFERENCE BETWEEN CSS AND PLA's?

Both services appear very similar in Google Shopping. The main difference between the two is that the customer is directed to a different checkout landing page.

As a CSS, Lyst places ads on your behalf through Google Shopping. A customer clicks on this ad and is redirected to your website. 

As a PLA, Lyst still features your products, however PLAs are placed by Google on Lyst’s behalf and redirects customers to the Lyst website. Customers are then able to checkout via Lyst.

Please see examples of PLA and CSS ads here.