Have you implemented an SDWAN solution in the retail sector with Palo Alto Network or Cisco Meraki? Do you have any feedback or guidance to share?  

CISCO Meraki is an excellent solution specially if need global deployment

I have implemented Cisco Meraki but not in a retail environment but there were small offices with service delivery. My suggestion is that you talk to your providers about your traffic flow and links to other locations. Understand your volumes of data usage at each location so that you can plan the capacity of the appliances correctly to allow for growth. If you are not traversing traffic to other sites then you may not need an SD-WAN at all, especially if your applications are all in the public cloud. Make sure they work with Cisco also to ensure best practice implementation as some providers will try manual implementation but Cisco has the option of profiles that makes it easier for future management. All the best.