Hi,
If I understood your project description correctly, what you want to do is the following:
You have a an invoice template, which you want to populate with several invoice data. Basically you want to bulk-create your invoices, by supplying a spread sheet with line-by-line data for each invoice you want to create. You then want to save that spreadsheet in some location according to some naming scheme (e.g. by invoice number or something).
If that's what you need then I'm certain I can achieve this within 24 hours (if not a few hours). I would write a script using perl (perl is a scripting language), which would take your data-sheet and invoice sheet as input files, it would then parse each of those files and output the relevant invoices for you as pdf documents.
You would need to install the perl interpreter, but it's very simple and I can help you do this. I'd also give you as many bug fixes and changes as you needed for this to work as you want it to.
If you'd like any more info, please just send me a message here :)
Best wishes,
John