Można pokazać klientom z USA jeszcze to, znalazłem na stronie  http://www.royalmail.com/service-update/international . Może po przeczytaniu tego będą bardziej wyrozumiali. 

North America (USA and Canada)

USA
Start date: 17th February 2015
End date:  Ongoing

Some parts of the South Coast are currently experiencing severe weather conditions. The worst affected areas are Tennessee and Kentucky. These conditions are affecting all mail services and as a result delays are possible.

USA
Start date: 17th February 2015
End date:  Ongoing

A number of factors have resulted in delays to mail processing at JFK, New York, these are: A change of Ground Handling Services for one of the airlines has caused a build up of mail. Continuing severe weather conditions along with a significant increase in mail volumes from China ahead of the Chinese New Year, has resulted in delays of at least 24 hours.

The USPS is working on processing the inbound mail volumes as quickly as possible, and has put a 24 hour operation in place to support this, as well as working with US Customs to find ways of speeding up the flow through.