Hi Samuel
The FM Andrew mentioned does work for Customer BPs.
For the GUI, there are some tricks to make it work, but I understand that you may not want to rely on "tricks".
What about Fiori automation? Have you looked into that? You would have to upgrade your Developer license to Developer Pro, but the runners wouldn't need to, and it's best to be on version 25.1+. It gives you more granular control on the action of add or update (aka edit) with the line item behavior, and there is no worry about positioning in the right place. Under the covers it grabs the right key fields and will perform the add or update regardless of where it is in the list - position irrelevant.

We also have a Studio for OData in early release, where you can call the published OData APIs that SAP provides, and it has API calls for pretty much all of the BP:
https://api.sap.com/api/OP_API_BUSINESS_PARTNER_SRV/resource/Email_Address
supplier: https://api.sap.com/api/OP_API_BUSINESS_PARTNER_SRV/resource/Purchasing_Partner_Functions
customer: https://api.sap.com/api/OP_API_BUSINESS_PARTNER_SRV/resource/Sales_Partner_Functions
If either Fiori (released) or OData (early release) is of interest, please reach out to your account team.
Best Regards,
Sigrid
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Sigrid Kok
*Precisely Software Inc.
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