We just returned from a month in the UK. In his London 2015 travel book, Rick noted that you could mail personal purchases from the UK to the US, limited to $200 a day in value. And you can. However, there are problems. On four different days, we mailed packages of books home, staying under the $ limit. It was expensive but we felt worth it since books add weight to luggage (both checked and carry-on).
Each package is delivered to US Customs for checking and then to the postal service for delivery. We used the mailing boxes provided by Royal Mail, and mailed from a Royal Mail post office.
Two of the four made the journey intact. The other two are another story. One box had obviously been damaged -- possibly ripped by a machine -- but had been slit open (we assume by Customs) and retaped. Only three of the 11 books it contained made it through -- the others were simply gone. (We're not talking rare books here -- but stuff you buy in bookstores and museum shops.) The other box had been slit open, but not retaped -- and it was delivered empty. Looking at the slit, someone literally had to hold the box on its side and shake it to empty it.
The box that was sent through LaGuardia Airport survived intact and undamaged. The other three were sent through O'Hare -- the two with missing contents and one that was damaged but somehow the books survived.
So -- the note in Rick's book is correct -- but after this I don't trust either the US Postal Service or US Customs. And now it will be weeks or months of claims discussion with Royal Mail.
Fortunately, it was books that can be replaced.