1) Free product level traffic from Google’s main search results is officially dead. However you can still receive free product traffic from shopping.google.com until October.
2) Google is going to start charging for clicks on shopping.google.com as soon as July 23rd.
3) PLA advertisers are getting a positional advantage on shopping.google.com over those who are not on PLAs.
4) PLA traffic is not converting as well as traffic from shopping.google.com. You should audit your PLA traffic to make sure you are not overpaying for lower quality traffic.
5) Bids are already impacting relevancy on shopping.google.com even though you are not being charged for that traffic.