While we all may have read bits and pieces of this in philatelic media and websites, I only recently read the entire account of this "audit" of what happens to off-sale items at the USPS Stamp Fulfillment Cneter, including discounted sales, inventory management, etc., etc.
https://www.uspsoig.gov/sites/defau...T-16-001.pdfIt is also of interest to note that as per the date of this document (06/15/2016) not only did the USPS sell Harry Potter stamps to Royal Mail at a discount but since they have yet to receive any payment for it, the current situation in the UK suggests that this will result in even a further loss to the USPS.
I found it an interesting read of what happens (or doesn't happen) behind the scenes at SFS.