When you have to pay duties on something, it is usually based on how much, how heavy, or how big it is, or what it is worth. You pay duties proportionately, which means you pay more if you have more of something and less if you have less. If the amount of duties is decided by a certain quantity, weight, size, or value, the duties are charged fairly for a bigger or smaller quantity, weight, size, or value, you can find more information about this by looking at the Customs and Excise Act 1986.