Pushover is a cloud-based simple push notification service that allows businesses to receive multiple push notifications from websites, devices and applications, including Android, iPhone, iPad, and Desktop (Android Wear and Apple Watch).

Key Features:

• Centralized user, group, and application management
• Higher free message limit (up to 25000 messages per month)
• Automatic group updating
• Priority message delivery that ensures your notifications arrive even faster
• Customized support for users via e-mail

Free trial, and just a 5USD one-time purchase.

Full pricing plan: https://pushover.net/pricing