Brevo PushOwl: Email,Push,SMS

The absolute best for push notifications.
Clean and minimal design ans easy for beginners to understand.

There is option for push notification for when a new customers signs up, push notification for when a items is on sale..etc.

And they also integrate with review app like Judge.me and back in stock app so they apps can send notification to customers via push owl.

A must have app for your website.