I bought these mailing envelopes (They are white) from Samuel French and I am wondering do agencies accept these types of envelopes; where you can see the headshot through the envelope?

Those suck! Those are by Keith Wolfe who makes those outdated "Right Agency for You," "How to Get Your SAG card," and "Where to Train" books. They're too bulky and the contents move around in them. The headshot especially slides around the window.

If you're going to use the see-through envelopes, get the ones here (scroll to the bottom):
http://www.breakdownservices.c.../actor_supplies.html

These ones are 8.25x10.25 so it snuggly holds your headshot in resume in place. Of course, cut your coverletter down to 8x10 so that fits.

DON'T seal these envelopes. Just fold the flap in. Because it's so fitted, you don't have to worry about the headshot falling out because the weight of the contents keep the flap in place. Of course you can always put a small circular seal where the flap and envelope meet. MAKE IT EASY for the agent to open!

I've used these window envelopes and got much better results than when I used the normal 9x12 manilla envelope.

Good looking out, I just ordered some "quick view" envelopes.

You don't need them and they don't help. They might take a look at your shot through the window, not open it and toss it- without giving your resume etc a chance!!

I say pass on them too. In my experience, they do not help at all.