UPDATE: After troubleshooting on postman, I've found that start_date and end_date are nessesary fields. Everything now works fine on postman with 200 code. However I am still getting a 400 error on zapier...
I've managed to POST my API credentials and generate an access-token from the previous step in my zap sequence using a Webhook. I then used this token in a GET Webhook in the next step to try and retrieve transaction information (PayPal processing fee, etc...) using transaction_id and fields queries. The transaction ID I am using is being pulled from an actual Woocommerce order on our store, where the customer decided to use PayPal (I've also searched the ID in paypal and it is valid).
The reason I'm doing this is to try and pull extra information from PayPal- Such as payment processing fees about the transaction that Woocommerce does not provide through their app on Zapier.
Unfortunately I am seeing the following error 'The app returned "Invalid request - see details."'
I suspect it is because I am calling for the information incorrectly. In my mind, I am using transaction_id as the unique identifier for the specific transaction, and fields to retrieve all information regarding the transaction.
Error code in Zapier: "Status Code 400 Bad Request" - PayPal docs state: "INVALID_REQUEST. Request is not well-formed, syntactically incorrect, or violates schema"
Update: If you use this method to pull transaction information, you will need to set a delay long enough where the activity time on your paypal dashboard switches from a time to a date (meaning it is the next day). If you don't do this, no important information will be pulled.
I've figured it out. If anyone has a similar issue, set it up like this (probably use more dynamic dates though):