Zapier

What is Zapier?

Zapier is a powerful integration platform that lets you connect Billsby to more than 2,000 other applications and services. This will allow you to synchronize actions between Billsby and other tools your business uses.

How can I retrieve & utilize custom fields through Zapier?

Since custom field responses are stored at subscription level, to retrieve and utilize them, you'll need to get the information from the 'Subscription', not the 'Customer'
Once you use an action to search for a specific subscription, you'll be able to see all of the information found, and even utilize it in a subsequent action.