That can't be right. The USPS site (see above) specifies no advertising (i.e. catalogues), but does not exclude invoice/packing slips. I don't see how they can do that.
I believe the justification is that invoices and packing slips have to be mailed first class. If part of the package has to be mailed first class, then all of the package has to be mailed first class (there are some exceptions).