If the package is stiff they may re-classify it as a package and delivery it with postage due. It needs to fit into the envelope sorters to qualify for the flat rate.

It's not just the less than 3/4" thickness that makes something a "large envelope". It has to be longer than 11 1/2 inches -or- taller than 6 1/8 inches and it has to weigh less than 13 oz without being rigid.

Sorry, rant removed.

A book is rigid so it doesn't qualify for the large envelope (flats) rate.  Your books should get reclassified as parcels somewhere along the way and arrive postage due.  Not saying that will happen but it may.

ETA:  I looked up the sheet I made for myself when they split FC into the 3 classes.  To qualify for the Retail Flat Rate, your parcel must be flexible, rectangular, and uniformly thick.  Also it must be not more than 15" long, 12" high, and 3/4" thick.  They have a whole test for the flexible requirement and a book just won't meet it unless it's an overly large paperback, like magazine size or better.

I have actually found in the past that most postal service employees are trying to help me find the cheapest postage available.  There  have been a few cases where they weren't as helpful as they could have been, but for the most part if something was available to be shipped a different way for a cheaper price, they offered it to me.

I can't keep up with postal regulations, so that's what I usually do.  If I'm using the APC I just go with whatever I had planned in the first place (though I wish they would do media mail there, ah well).

Mary, just bring a list of the different MM rates, weigh your book and then buy the appropriate stamp ($2.13 or $2.47 or whatever) from the APC machine.  This is what I do and it saves a lot of time

As Leslie suggested, you can use the 'Buy another denomination' button on the APC machine for media mail, Mary, as long as you know how much it weighs! Just punch in the amount and it will print that denomination for you!

Thanks for the info, I had been wondering if I could use the APC that way.

My post office wouldn't let me use the large envelope rate for a book either and it was a thin book.  That's more for documents.

I've caugt a lot of packages dropped in our slot where the customer used large envelope rate for things that obviously didn't qualify. I mark them "returned for postage," and the amount need to bring it up to parcel rate, then send it back to the return address. If I don't catch it then, someone else is likely to down the line. Better that they get it back from me the next day than a week later from someone else.

I specifically asked this at my post office shortly after the rates changed and they also said that a thin book did not qualify.

The deciding factors are height, weight, thickness and the one that gets most books rigidity.  The machine that sorts large envelopes will send the items around a drum like object if it will not bend around this it is a parcel.  Most books because of the binding are not going to make it around the drum.  The exception is some very thin (usually children's) books.