I send out FIFO books using DC-only.  Lately I've had a problem with books that weigh more than 6 oz but less than 13 oz.  When I print the labels, PBS offers me Media Mail as a default, usually for $2.69 as the estimated postage.  When I take those books to the PO, the scale assigns them to First Class Small Parcel (or occasionally First Class Large Envelope, depending on the dimensions), which includes tracking at no cost.  However, the postage for First Class is often more than $3, i.e. more than the cost of what PBS indicates for Media Mail plus the DC fee.  Once the scale has determined that a book should go First Class, the prepaid DC becomes a waste.  On the other hand, if I tell the counter agent not to scan the bar code and just to go by weight, then the scale will happily offer Media Mail as an option, but the bar code won't scan.  Then I have the option of buying tracking from the PO for an additional $1.05 and (again) wasting the prepaid DC.
Has anyone else seen this problem, and is there a way around it?  Is it possible that the tracking bar codes really are specific to the class of mail that is chosen when the label is printed at PBS?  And if the PO will send a book between 6.1 and 12.9 ounces for either First Class or Media Mail, then why doesn't PBS offer me the choice of which kind of label to print?  I am wasting enough DCs that it is starting to add up.

Several items here Clair.  Tracking barcodes have been somewhat problematic for a couple years now.  They ARE valid, but USPS updated their software and most counter clerks have not been trained how to scan pre-printed barcodes into the transaction....especially Media Mail.  The problem is merely the counter scanner/software.  All other USPS scanners read PBS/USPS Tracking just fine.  If you use printable postage or even APC (stamp option b/c APC doesn't offer Media Mail) your package will pick up all enroute and delivery scans...or lucky enough to find a counter clerk than can properly process your valid shipping label.

Yes, IIRC the rate is now encoded into the Tracking barcode.  And again, that's part of the scanning problem, as clerks attempt to enter rates and other data before scanning the barcode (which confuses the software and flags the transaction/barcode invalid).  If they merely enter the zip code then scan it should go in (PBS has a help document on this issue, which can be printed and used at your USPS branch).

First Class vs. Media Mail, IIRC the break is at 7 oz.  Less is cheaper 1st class and heavier is cheaper Media Mail.  If you are having issues at the counter, the problem may be incorrect weights in the PBS database causing labels to print with an incorrect (automatic) rate.

And finally, if you print Tracking only and purchase postage at the USPS counter (such as I did for several years)...your Tracking is not pre-paid.  You have paid a PBS fee for printing and on-site tracking (credit assurance/quick credit).  The actual .23 for USPS pre-printed Tracking is charged at the counter (or add it on manually when using APC).

Denise, thanks for your input.  The problem is definitely not the counter personnel in this case.  I have been using the same PO for years, the counter staff have seen all the help docs available to print out here, and they bend over backwards to scan my books correctly because I am a frequent and heavy PO user (and because they are just really nice people).  They know to enter the address info and then scan.  There are never problems when I mail a HB or other heavier books that "should" go for Media Mail class - it's strictly an issue of low weight books.  So I wasn't questioning the validity of the barcodes, but rather their accuracy.
Yes, I do understand that the 23 cents is the fee to actually buy the tracking - I should have said preprinted, not prepaid.
Thanks for validating my concern that the problem may be coming from incorrect weights in the PBS database.  That was my impression as well, and I'll start reporting individual instances now.  What's the best way to do that?

If that's the issue, then yes the weights can be off.  One of the reasons I buy counter postage, so staff can weigh and dispense correct postage.  Usually the staff confirm the weight/suggested postage on my pre-printed labels.  But once in a while one comes up different.

Unfortunately, I don't know how to correct weights in the database...or even if they do.  Could be a matter of different printings.  You could bring it to the attention of either a member of the Book Data Editing team or directly to TPTB.

Glad to hear your local postal staff is on top of things, we so often hear differently