Should I Just Use 29c Envelopes?

I believe the "letter" stamps that don't show their denomination were issued for domestic use only. Check a "G" stamp.

Maybe without showing a value, they don't meet U.P.U. requirements.

Yes, all the "letter stamps" issued during rate changes are valid for domestic use only.

Well glad I didn't waste a letter stamp on it then...

I handed it over the counter because I wasn't sure of
the postage needed going to Canada. I asked him and he
said "well, let me count these up". I said "90 cents".
He said "I'll take your word for it, that'll be 4 cents".

So it was 94 cents total.
He started to run them through a machine, but noticed the
old stamps and said "You got a nice stamp 'thing' going
here." and then he lightly hand cancelled them instead.

Pretty nice of him.
They are cancelled so lightly though, I don't know if
they got the postmark a nice cover deserves. Oh well.

Ratio:

Well, it is nice you got a postal clerk that appreciated the stamps on the cover and cancelled them lightly! Good for you!

David