I don't see the CPES Certificate as a certification. I see it more as an ownership mark. It's something that identifies the stamp as being owned by a collector/dealer of fine stamps. It describes the stamp like a Lot Description.
VGG Certificates are the best, however, I have currently seen many that do not point out faults in the stamps. sometimes major faults. If you want to spend $40-70 on certificate then that's up to you. The VGG Certificate does increase realizations but the amount of the increase maybe very little on a stamp with a hammer price of $200 or less. If you have a truly expensive stamp then the VGG certificate is the one to get. You may have to put up a fight to get the correct opinion.
Both CPES and Gratton (AEIP) certificates are not recognized by better Auction houses in Canada. The difference is the AIEP certificate is expensive, whereas, the Dealer gives you the CPES certificate with the purchase of the stamp. I see almost no value in the Gratton (AEIP) certificate. When I sell high value stamps at auction, they just toss CPES and Gratton certs in the garbage. I hate to say it but that's true.
A peer-reviewed article in a philatelic journal is probably better than a self- generated cert.
I dislike the overall tone of this thread so far, a little community respect please:
Richard Gratton, an AIEP accredited philatelist, has 40+ years experience in pulp and paper. He has also published several peer-reviewed articles on paper in the Canadian Philatelist. Ariel Hasid has supported and dealt in high-end BNA philately for decades.
Perhaps readers should know this before accepting postings on a chatboard as truth.