Can I save a push notification and send it again later?

Yes. In Plugins → Push Notifications, use the Notifications list:

1. Fill in Title, Message and a Link (for example /category/new-arrivals).
2. Click Add notification. It's saved with the trigger Manual.
3. Whenever you want to send it, click Send now on its row and confirm. It goes to all your subscribers.

Use Edit to update the wording before each send, or Delete to remove it. For a one-off message at a set time, use Schedule a message instead.