Start here:
https://goscf.com/t/41467&whichpage=2If on cover it is worth a certificate to remove all doubt.
I went through this issue with a cover far more scare than those listed in Scott. It was a 498f single with a 3 cent paying the parcel post rate of one cent per ounce on a four ounce parcel from AEF France. For me the cert is to clearly establish the stamp and perhaps I can get such a usage added to the listing. That said, this is one of those items you need to see a lot of to make the 498e/498f determination.