1976 postmarked $2.00 bills - info needed

First of all I know nothing about collecting paper money thus this question.

We were just going through some of our files and found 50 crisp two
dollar bills with consecutive serial numbers in an envelope. Each bill
has a $.13 state stamp on the bill (each stamp is a different state) and
they have been postmarked April 13, 1976. The postmark overlaps both the
stamp and the bill. I vaguely remember that my then assistant, who was a
coin/money collector, was going to go to a bank and have this done for
her so I gave her a hundred bucks and said "do the same for me", which
she did.

Well, that was over 30 years ago and we just discovered that I still
have them. I have done several searches on the Internet attempting to
find a value and have come up with everything from $2.00 each to $35.00
each thus I suspect that the real value is somewhere in between.

All in all, if any of you experts could give me a "ballpark" value for
the set it would be greatly appreciated.

Happy New Year,

Don