The average paperback costs $2.38 to mail out. Can you print out a $2 stamp from the APC, add a forever stamp and send it on its way? I am not sure if you can mix printed postage plus a regular stamp. Obviously I know this will put you a few cents over but I have an abundance of forever stamps and this seems to be a good way to get rid of them.

yes you can do this.  I've added stamps to the printed postage from here before when I've forgotten to adjust the weight.  They can track you from the APC stamp for packages over 13oz.  I just periodically buy some $1 stamps and some other denominations for when I'm mailing smaller books and don't feel like going to the post office. I have never used an APC machine-I've never even seen one. But I've seen people post on here that you have to create a stamp to get a denomination you want. 

The only APC restrictions are that the minimum initial transaction must be at least $1.00*, and you can only buy 5 stamps per "transaction" (other than a sheet of 44's). So, if you wanted to get a couple of .39's to have on hand in future for the extra postage for the odd package just over a pound, you'd have to buy a $2.38 as the initial transaction (for the theoretical book you want to mail now), and the .39's as a second transaction.**

* the "buy stamp" open will not work for .99 or less at that point

** the minimum stamp value should now show as .01

That is exactly what I do with a lot of my APC stamps. I get a bunch of $2, because most FC or MM will be over $2, and then just add a combination of other stamps to get to the total I need. I have been doing this for over a year, just tossing them in a blue box or the bin by the APC and never had one returned for being over 13oz or anything else.

That's what I do as well, except I tie up the machine for a while because I buy about 10 $2.38 stamps, 20 $1 stamps, etc. Once you insert your card, you can keep going, but it is in 5 stamp increments. I do the stamps by mail thing occasionally to buy a ton of assorted value stamps .01, 02, 03, .10, etc., so that I always have postage on hand that will add up to the proper rate. I don't use my "forever" stamps if I can avoid it, the rate will be increasing, and I don't want to give them more $ per pkg than necessary. :-) My carrier picks up my stuff, over or under 13oz when they have APC.