I filed FOIA requests with USPS, which they ignored. The metrics are done by a private third party, claiming the data is propriety (privatization allows govt to hide uncomfortable truths).
I got the numbers client lawsuit against USPS. Bulk mailers do their own metrics / tracking (using test mailings). They claimed USPS's "UAA rate" (undeliverable as addressed) was higher than claimed, so they shouldn't be charged as much.