We have 2 blue boxes in front of our post office. One for stamped mail, one for metered mail. They are drive-n-drop boxes.

Inside, an APC with a bin for parcels, etc under 70 lbs. This is where I normally drop off my APC/Paypal postage-d books.

QUESTION: If Paypal postage is considered metered (thus, traceable) postage, can I drive-n-drop my Paypal postage-d books in the blue metered bin???

I'm not a "USPS expert" (nor do I play one on TV), but I've always understood Paypal-postage to be considered metered. I would try it, but if you're nervous, maybe with a non-critical package.

Disclaimer: I don't work for the post office, but I have successfully mailed Paypal postage packages for PBS and eBay by dropping them in the "metered" receptacle.

Yes, you can drop them in the drive-up boxes as long as they'll fit in.  With Paypal postage, you're a "known" mailer.  They can trace the postage back to you so you can deposit it in any dropbox or the APM.