it is very hard to go through UK customs
You go through customs when entering or leaving a country in Europe.
No because Canadian flights to Mexico go north over the north pole and over South America not south over the USA, {Just Kidding} no you do not have to go threw US customs on a flight from Canada to Mexico or vise versa.
NO your answer here...
Declaring it as a firearm is the only sure way to pass it through customs.
No, imported mattresses do not have to go through customs because they are not considered a hazardous material.
You go through customs when leaving the US before boarding your international flight.
When flying back to Canada from Nicaragua via the US, you will clear customs in the US at your first point of entry. This typically occurs at the airport where you land in the US before your connecting flight to Canada. You will go through U.S. Customs and Border Protection, where you will present your passport and any required documents. After clearing customs in the US, you can continue on to Canada, where you will go through Canadian customs upon arrival.
to the airport
When leaving the US, you go through customs at the airport before boarding your international flight.
You typically go through customs when flying to Europe after you arrive at your destination airport.
When flying to Europe, you go through customs at the first airport you arrive at in the European Union.