The post office has a scale you can use to weigh your envelopes and contents. I use it all the time and it takes the guesswork out of it. I even use it just to see how much something weighs (or rather how much postage I'll need), then ship at a later time.

Under 2 ounces.

That 13 ounce limit really seems to be more a guideline, I've more than once thrown things in there that weigh over a pound.

About how much do you think a 9x12 envelope, an 8x10 picture, another 9x12 SASE envelope, and a letter on plain old notebook/binder paper would weigh?

I do not send out many autograph requests, and when I do I usually just do it from the post office. I used one of those ehh... whatever... mail boxes? Big blue ones. Apparently you can only have 13 ounces. I wasn't aware of this until I had already put my letter in and read the sign in the front -_-. Massive fail on my part.

I was just wondering how much 13 ounces exactly is, I have no idea, would this fall under it?