There would need to be a PATCH (to update an existing batch) available for ARPostReceiptsAndAdjustments which there is not. Just as an FYI, in your code above you were trying with a POST. You can see all of the available calls in Swagger on the server where Sage Web API is installed.

There would need to be a PATCH (to update an existing batch) available for ARPostReceiptsAndAdjustments which there is not. Just as an FYI, in your code above you were trying with a POST. You can see all of the available calls in Swagger on the server where Sage Web API is installed.

PATCH is added in v2021
www.sagecity.com/.../how-to-make-web-api-ar-invoice-batches-post-able

The question here is appending (PATCH) to open AR Receipts batches, not the ability to post the batch via API.  

http PATCH is to update records. Adding new invoices into an existing batch requires PATCH and changing the Batch status to ready to post also requires PATCH.
I only provided the link as confirmation that an upgrade to Sage 2021 might be an immediate solution if adding invoices to an existing batch is a requirement.