Hi there, Candace.
I don't know you, and you don't know me (or anyone else on the message board).
However, let me try to give you some information about your stamps and some places you can go to verify the information so you can draw your own conclusions.
First of all, the Wells Fargo Pony Express stamps were all single color. It appears that one of the stamps you photographed has the denomination in a different color than the other stamps. That would point it to being a facsimile of the real stamp. If that is an artifact of the photograph, and it is a single color, then it has an outside possibility of being genuine.
This series of stamps is listed in stamp catalogs as 143L1, 143L2, 143L3, 143L4, and 143L5. As mentioned, they have been reissued and reproduced a number of times after they were originally issued. If you look at a lot of these, as many of us have, you'll see that the margins on most of them are very close to the sides of the images. Only on the sheet margin sides are there even the chance of having margins the size of the ones in your photo. Again, it predisposes people to think that they are facsimiles.
Even if they are genuine, you'll find that they are not "priceless." The 2010 stamp catalog I have lists the five stamps I mentioned as being worth:
143L1 $2 red: $ 175 unused, $ 800 cancelled (with a Wells Fargo cancellation)
143L2 $4 green: $ 300 unused, $1,250 cancelled ( " " " " )
143L3 $1 red: $ 90 unused, $ 750 cancelled ( " " " " )
143L4 $2 green: $ 250 unused, $1,750 cancelled ( " " " " )
143L5 $4 black: $ 175 unused, $6,000 cancelled ( " " " " )
(I don't buy a new one every year - the prices don't change that much)
The fact that the person who "authenticated" these stamps told you that they were "priceless" is another warning sign. This is basic information that any reputable appraiser should have had readily available. As they completely misstated the value of the stamps, that's another warning sign to those of us on this board that the information provided to your grandfather was of limited (at best) value.
Given all of these things, the warning bells have to go off and the responses you received are based on that.
As for outside validation of this information, I'd recommend that you go to to Siegel Auctions website at
www.siegelauctions.com At the top of the screen, you'll see a choice for "Power Search."
Go to that page and put the catalog numbers I provided into the box that says - "Catalog# Equals"
Hit search and see what the single stamps look like and what they have sold for in the past.
Siegel is probably the most reputable auction firm for this kind of material.
You might notice that there are not many examples of these stamps having been auctioned (in unused condition). This is not a function of their rarity, but more of their commonality. They just don't get enough to be worth lotting as individual stamps.
Sorry to break the news, but I would not set your hopes too high for getting much in the way of a tax deduction for these, should the Smithsonian even want them (they have A LOT of pony express material there).
Good luck with the rest of your grandfather's collection.
C.