okay I just printed my first custom label (its going first class on the place where the stamp goes  it says visit    the post office for the correct  postage;

now I was wondering if i went to the APC and bought the postage +.18cents from the APC would that work?

as we know we can get stamps in any amount that way

Yes, that's exactly what many members do in fact.

Yes you can do that, but keep in mind that the APC requires at least a $1 purchase.  
Post the book as if you were going to buy all the postage on the APC  then just before the point of purchase there will be a "Less Stamps" button where you can enter the postage you already have on it (or what should be on it to leave 18 cents). Then machine will then make you buy a couple more stamps  to make it up to a dollar,

Alternatetively, you can buy a $1 stamp first  then specify your 18 cent stamp. I think this would work.

Hope this helps!  :)

Because you can't buy eDC through the menu on the APC, you will need to take a few more steps than just putting it on the scale and following the menu selection. 

What I do is weigh the package at home and look up the postage on USPS.com and then add the .18 so I know the total postage I need. Then at the APC I just use the buy a different stamp option to buy one stamp for the total amount.

If you don't have a scale and can't get the weight of the package any other way than on the APC scale, then I would go through the menu for First Class and get your base postage that way. Tell it you want the little label not the big one it defaults to (at least ours want to print a big old label). Then when it asks if you want to do anything else, say yes and use the buy a different postage stamp option and print the .18 to add to the package. As long as you print the main postage label in the same session as the .18 you should be fine on the $1 issue.

thast what I did today I had to mail two packages anyway)