Why am I getting shopping trolley notifications about local store savings?

This notification isn't from Google Maps at all, but from Google Assistant a.k.a. Google Now. All the notifications with the G icon are from that app. It's not controlled by the store or targeted to you: it's just about the place that Google thinks you're at. The location update rate isn't very fast (to save energy use), so sometimes you only see the notification after you've already left, or just if you go past the place.

To control it, start the Google app or swipe your homescreen left to open the Google Now page. Then press the "hamburger" icon (looks like ≡) to open the navigation drawer, and choose Settings. From there, scroll down to Your feed and click that. On the next page, scroll down past Get notified about to see a list of all the types of Google Now card you can get notified about. Places is the one about nearby places. You can also turn off all the notifications at once on this page.

Note that turning off the Places option will also hide the similar notifications about restaurant and bar reviews, timetables when you're in a bus or train station.