HI Andy,
You can investigate Sample document (F-01) or reoccurring document (FBD1 to create and F.14 to post) functionality. These should help if the invoices to be posted are similar.
If they are different, you can talk to a developer to create a program, there are standard programs for reading excel files and the bapi BAPI_DOCUMENT_CREATE will create the documents for you.
Regards,
Peter