Hi Mike, hope all is well and would forward to catching up soon.
You are right on the measurement from the USPS website, a C5 is 2/8th of an inch over their "small letter", a really small amount but still over. Sometimes, when i bring a 6x9 to the counter, they will hold it up to a chart to measure size. I seam to remember the last time they did that there was a little room to spare. Most of the time they don't bother to measure and just pass it through but I guess you always run the risk of it being pulled back.

Hi Jrott, how are you doing? Was just thinking about you the other day, I'll pm when I get a moment :-)

I checked the USPS website measurements against the USPS website a few weeks ago and measured an envelope with my trusty ruler, it was a couple of millimetres over the size for cheap postage on their tables.

I usually send 8x10s to the US, but have been thinking about trying to track down some smaller envelopes so it all gets a bit cheaper (just busy/lazy/whatever!)

When I do my mailings to the UK, I use a 6x9 inch envelope for mailing and the return. My typical mailings are

6x9 inch envelope that includes one 5x7 photo, letter, rail ticketand the return envelope. This weighs in slightly under one ounce so it costs me a 98 cent stamp
If I send 2 photos, it weighs in "slighty" over one ounce so it would cost me $1.76 or two 98 cent stamps
Converting size for you, a 6x9 envelope would be almost a C5
C5 envelope would 229x162mm = 9 x 6 3/8
As Stan said, the C5 is slightly larger then the 6x9 so it might make a difference in cost, next time I am at the post office I will ask them

What size photo are you sending, and in what?

I am a well-educated Brit, but find the USPS website quite baffling, and it doesn't help that US envelope sizes quoted for on the USPS website are not the same as those which are readily obtainable over here in the UK.

As far as I can make out (from the ones I've sent and got back), if your reply envelope with its contents is small enough to fit a 6x4 photo quite comfortably, (i.e. thin and light), then you need about 98c in stamps.

If you are sending a C5 envelope, they are slightly above the standard size for the US, and I've found that 2 x 98c stamps and a Forever stamp (which is currently worth about 47c or thereabouts - I'm sure that anl American could correct that if it's wrong), will work. A C5 envelope is slightly over the size where US postal rates jump from the equivalent of our small letters up to large letters.

For a C4 envelope, with a couple of photos and a piece of moderately thin card for protection, I put on 3 x 98c stamps, and have always received them back without a problem.

If I am wrong, I would, like you, welcome some guidance from a helpful American, but until then that is what I have managed to glean from a lot of searching around on here and the USPS website.

I don't do IRCs, they are a very expensive way of buying postage, and inconvenient for senders at both ends.

US stamps, of course.

I am from the UK and want to write for an autograph in the US. What do I put on the return envelope for postage