Try MS robotic automation (they may have a cheap / free account)
https://powerautomate.microsoft.com/en-us/blog/process-invoi...
using an AI to go through all the files and do the math for me.
The only way you could rely on the the result would be to manually check each invoice, given all of these tools tendency to hallucinate. Might as well just manually collect the values at that point.
It seems to me you could use a spreadsheet and do this by hand in an hour or two.
Would you really trust any of these LLMs? I wouldn't.