The documentation for the BrickLink API PushNotificationMethod suggests that the data sent to the URL you registered on the BrickLink API Consumer Registration Page is sent to this URL (via a POST verb, by the way) and as such you don’t need to call Get Notifications. Given the body of the POST is empty, this is not right – what you instead need to do is use any POST to your registered URL as a prompt to call Get-Notifications. It’s probably best to periodically call it too, given “it does not guarantee delivery of all events” and doesn’t either based on my experience.

A notification to be created when:

• Order
  • You received a new order.
  • Buyer updates an order status.
  • Items of an order are updated (added or deleted).
• Message
  • You received a new message.
• Feedback
  • You received a new feedback or reply

Also note: NULL fields are not included in the returned JSON. Some fields names don’t match the documentation (eg: drive_thru_sent instead of the documented sent_drive_thru).