Certification will get you the highest sale price. Certs will confirm the presence or absence of faults, such as thins, re-gumming, or re-perforating. They will also answer the hinged / never hinged question. I also have a set of these - MNH and PSE certified XF90. Here's an example from my collection.
PSE will charge you 5% of catalogue value or $30 - whichever is larger. Grading is optional and there is no added charge for it. Expect it to take 30 working days to get the certificates once they get the stamps.
C13 and C14 will cost $30 each
C15 will be $42.50 (5% of $850)
subtotal = $102.50
priority mail return shipping = $15.90
$1000-2000 insurance = $14.00
overall cost estimate = $132.40
That is 8% of the cat. value (as VF80)
When I submit stamps to PSE, I use Certified Mail, plus they must sign for upon receipt. I have private insurance through APS that covers any shipment I make. I do keep a scan of the stamp(s) and the PSE submission form as proof plus the CM receipt and signature confirmation of delivery. YMMV.