See the API's various operations for details on their request parameters and response data:

OperationMethodEndpoint
Invoices
List invoices and credit memos for a contractGET/contracts/​{contractId}/​invoices
Download an invoice or a credit memo fileGET/contracts/​{contractId}/​invoices/​{invoiceId}/​download
List invoices and credit memos for an accountGET/invoices
Notifications
List notifications for an accountGET/notifications
Create a notificationPOST/notifications
Remove a notificationDELETE/notifications/​{notificationId}
Get a notificationGET/notifications/​{notificationId}
Modify a notificationPUT/notifications/​{notificationId}