All Advanced Billing outbound requests (i.e. Webhooks, gateway requests, etc) are currently sent through the following addresses:
50.112.130.129
50.112.144.200
50.112.147.117
107.23.118.166
107.23.119.26
107.23.118.227
54.229.13.248
54.229.57.150
54.229.48.92
You may need these for your firewall or for your payment gateway configuration.
Note: this list is kept up-to-date - this post is edited in place when we make an addition.