In regards to Pothi Sahibs I would NEVER recommend to send via mail. If you view SIkh history then you will notice how much resepct Pothi Sahibs were given even before Gurgaddi of Sri Guru Granth Sahib Ji. Even when collecting the compisitions of bani the scribes paid lots of respect to the pothi sahibs. So Sri Guru Ji says one is a tankhiya if pothi sahib is placed in a disrespectful place. In the mail a pothi sahib goes on the floor. If you know someone coming from the country where the book is then they can personally bring it on the plane and keep it respectfully on them.
ਜੋ ਸਿਖ ਗਰੰਥ ਸਾਹਿਬ ਜੀ ਅਥਵਾ ਪੋਥੀ ਸਾਹਿਬ ਬੇਅਦਬੀ ਜਾਹਗਾ ਰਖੇ ਸੋ ਤਨਖਾਹੀਆ - Sri Guru Gobind Singh JI
ਜਿਨ ਭੈ, ਆਦਾਬ, ਨਾ ਬਾਣੀ ਧਰਾ ਜਾਣੁ ਸੋ ਸਿਖ ਨਹੀਂ ਹਮਾਰਾ- Sri Guru Gobind Singh Jee
In regards, to books I have a more libearl opinion. Even though religious books should be treated respectfull; nevertheless ;religious books dont have the same status as Pothi Sahibs. Unless one lives next to a Sikh book store people cannot get access to certain required religious books so for thisr reason I dont see no harm in ordering such books. Even though one should first make an attempt to get certain books locally still if not possible I dont think its wrong to order, since reading religious book is not only helpful spiritually, and is a requirement for every single Gursikh.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥