it is currently possible to update shipment notes and package notes via API, despite the fact that it is not documented (see further down in case it helps anyone before they update the documentation)

i would like the option to append to notes rather than update notes, that way i could avoid an extra request to get the current notes first

just in case it helps anyone, here is how to update shipment and package notes
Method : POST - https://inventory.zoho.com/api/v1/packages/{package_id}/notes

Parameters Required : authtoken, organizationid and packagenotes.

Method : POST - https://inventory.zoho.com/api/v1/shipmentorders/{shipment_id}/notes

Parameters Required : authtoken, organizationid and shipmentnotes.

The maximum length of the notes allowed are 300 characters.