Importing Invoices

Want to get started by importing year to date Invoices from excel. Looks like it could be done using API. Before I potentially re-invent the wheel by developing a program to do this, has anyone already done similar?

I’m not aware that anyone has made a “generic” importing tool, although I think a number of users have created their own import routines for specific software packages.

Thanks. Looks like I may need to develop my own routine for now.
Looking at the API I notice that a key feature of the authentication aspect requires a “Submission Number” which follows on from the last one used, possibly in a previous session. Is there a method by which one can retrieve the latest number?