Hi @shaunhaage,
If you’d like to understand more about how the Pipedrive API works, we have entity-relationship diagrams on our About the Pipedrive API documentation page that could help with that.
As for searching the deal stage by querying our API with the last name field and a custom invoice field, perhaps you might want to try out the ItemSearch endpoints?
I hope this helps. Please let me know if you have further questions. 