I mailed 3 books several hours ago, with pre-printed postage on 2.  The "instant credits" for the 2 have still not come through yet. 

Not that it is really any problem for me.  But this is not how it normally works.

Were the books going to the same person but were put under seperate orders?  If so and you mailed them in one package using the printed postage than you will only get the instant credit for the one that's under that label. You'll get the other 2 when they're marked received.  It'll only give you the credits for all if it's on a bulk wrapper. 

If that's not the case then I don't know what to tell you. If you used the printed postage, marked them mailed and the credits still don't show up-try logging out and then back in again and see if that shows them.  If they don't show up after a bit, I'd contact PBS. 

Is it possible that you marked them mailed before you purchased the postage? If you forget the postage and reprint a wrapper after marking it mailed, you won't get instant credit. You do get a credit when the barcode is scanned though.

You should also double check that you purchased postage. I've "temporarily" changed my settings before and then forgotten to change them back. You can see in your Money Registry whether you actually did purchase printable postage.