So I just found out, pretty late into implementation, that Alternate Sales Amount (ASA) is not supported on Cash Sales or Invoices, only Sales Orders.

This despite the Help text under (SuiteFlex (Customization, Scripting, and WebServices) : SuiteScript : SuiteScript Reference Guide : Supported Sublists : Items Sublist) that specifically says that the "altsalesamt" field is supported for Cash Sales and Invoices.

So, the problem is that I am trying to pay commissions on a multiple of certain recurring items, and these items are recorded directly as Cash Sales - no Sales Order. We can't change the workflow, because the cash sales are entered via webservices.