Did you know that the 999 line limit has been solved?
Our best developers in Germany have come up with solutions to post FI and MM documents (among others) with more than 999 lines.
This feat is achieved vi the BAPI technology, not through online transaction processing.
For GL documents the BAPI is BAPI_ACC_DOCUMENT_POST
For MM-LIV documents the BAPI is BAPI_INCOMING_INVOICE_CREATE
Simply search OSS using one of the following keywords to find documentation:
Incoming invoice, maximum number of line items in an FI document, 999, F7 727, newGL, new G/L, General Ledger Accounting (new), PRCTR, KOSTL