Federal solar tax credit - proof of install?

So I know the solar tax credit question has been answered here numerous times. My question is more about documentation / proof of install: my system was installed fully late November 2025. PTO will not be done by the end of the year.

I keep trying to get the solar company to provide me some documentation about proof of install but they keep dragging their feet. They said this is "new to them" since previous years did require PTO before the changes in 2025 so they don't just have a document they can provide (since the PTO documentation was previously the proof). But they keep saying they are trying to get an answer for me (albeit slowly).

How have others been dealing with this? What actual documentation do you have to prove date of install? I don't think this is required for the actual tax form but I just want to make sure I am organized in case of an audit in the future. I have some unofficial emails claiming proof of install on a date but I was hoping for a more concrete invoice or something like that.

Thanks!