Do you care more about accurate valuation or accurate PPV's ?
In case of subcontracting you can't have both, but an accurate valuation should have priority, especially that there is no standard SAP report for PPV, which means you may need to revise the logic in the customized report.
For an accurate valuation you have to have the packaging material as component in the BOM of the FG so that its price is part of the calculation of the FG price.
Since the price for the subcontracted material is service + component costs it results in the fact that the component costs will end as PPV in classic PPV figure.
For the same reason SAP does not execute any tolerance check for this scenario, see KBA 1972920 - Tolerance check for price variance doesn´t work for subcontracting purchase orders