You should also keep invoices showing date of replacement and cost - many courts won't allow you to "upcharge" when you have to replace. I generally keep one copy in my payables file, one copy in the unit service file, and one copy in the tenant file (first tenant to move-in on the new carpet)...that way I don't have to dig for it at move-out.
We also update our PM system unit notes with date of replacement, cost, vendor, etc.