I gave my mailbox this $29 smart upgrade and it's a total game changer
Are there alternatives to using Amazon Sidewalk for the sensor?

Yes, there are alternatives to using Amazon Sidewalk for the sensor. You can use either a Ring Bridge or a Ring Alarm Pro Base Station to add the Ring Mailbox Sensor to your home network if you don't feel comfortable having your devices connected to Amazon's low-bandwidth wireless network.
How does the antenna enhance the Ring Mailbox Sensor's functionality?

The antenna enhances the Ring Mailbox Sensor's functionality by extending the device's range and avoiding interference issues that may be caused by a metal mailbox. It allows the sensor to communicate with the Ring Smart Bridge or other Amazon Sidewalk-enabled devices, ensuring reliable notifications when the mailbox is opened.
What is the cost of the Ring Mailbox Sensor at Amazon?

The Ring Mailbox Sensor costs $29.99 on Amazon. It is a smart device that sends alerts to your phone when your mailbox door is opened, helping you keep track of mail deliveries.