I have taken to writing the details on the outside of my SASE's which are on the edge of the limits. I often expect an a4 card back SASE to weigh close to the UK limits of 100g for the first price range. So under the stamps I write £1.10 2nd Class Large, that way they have no excuse for overcharging it as if they have doubt they will (or should anyway) check it to be sure. One came in with an overcharge, I went to the sorting office and invited them to weigh it before they handed it over to me, voila!

For 2 oz I always use 0.70. Every once in a while you get someone at the post office that doesn't know what they are doing. 6x9 is right at the edge of letter sized envelopes so someone new might have thought it was supposed to be charged as 'oversize'.

Apparently I have been estimating my postage all wrong. I just got a response back in my 6"x9" letter-size SASE (which had one forever stamp), and it had been stamped "Amount Due: $1.83". Luckily the person on the other end was kind enough to pay the postage, but I certainly won't let that happen again.

My question is, what is the best way to estimate the postage? I usually weigh the sending envelope with contents (usually just under 2 oz.) and then again without the inner envelope and use that for the SASE weight (in this case it was 1 oz.). All I had inside were a couple pictures and a lightweight card backing. Using the USPS website calculator, it should be between $0.49-0.70 for 1 ounce., $0.70-0.91 for 2 oz., $0.91-1.12 for 3 oz (depending on whether you check "contains a rigid object.")

So I'm confused where they pulled $1.83 from, that seems a little exorbitant for a letter size envelope.
This is the first time this has happened, and I have got many more successes using this same technique. Although now I'm wondering if this is why I've been having so few lately??