I do not send any paperwork with the birds when I ship them that people can access. If the person that bought the birds tells me they need NPIP papers, then I do include them in an envelope, taped tot he inside of the box. I have shipped to lots of states and have not had a problem yet. I have a bigger problem with areas that the express will ship to, some places cannot get Lives from Denver airport so I cannot ship there, you can check out yours on the USPS website under Lives. It would actually be Fed Ex doing the flights. For example, I cannot ship to Utah (most places).
I cannot however, ship birds to places if the person requires a health certificate, I know Washington State was or is doing that, my vets around here will not give a health certificate. That being said, I have never heard about a inspector being at the post office to regulate paperwork on incoming lives. I leave it up to the person receiving to check their state law requirements first before bidding. I have one lady from Illinois interested in my silkies on Eggbid, and she was told she needed a health certificate. However, when I chcked on various places regarding that, I saw it listed that she needed one for Exotic animals and birds, (meaning from another country) but not here. It also stated that NPIP papers were acceptable for the US birds. Most places will accept NPIP papers in Leu of Health certificates.