If you are on Android, you can use Notification Managers which can save notifications. I'm not sure which is the goto Notification Manager now, but I used to have LightFlow (https://play.google.com/store/apps/details?id=com.rageconsulting.android.lightflow) for managing notification LED and if I remember right it can store notifications although I didn't use it for that purpose. You would be better off searching for a latest notification manager or may be use a Tasker recipe like this (https://play.google.com/store/apps/details?id=com.balda.notificationlistener&hl=en) to filter offer notifications and store it as a note.

If you are on iOS, there's nothing much you can do as there are no 3rd party Notification control APIs and so you are limited to not dismiss notifications which you may need later.