To regulate the importation and exportation of goods in a country
Customs is a government agency that regulates the flow of goods in and out of a country, as well as occasionally other related services such as taxation and border patrolling. When Hermione Granger ordered a Broomstick Servicing Kit for Harry Potter's thirteenth birthday, she was concerned of how to send it to him, as she feared it would be opened at customs if she sent it via traditional Muggle channels, such as Muggle Mail.
Behind the scenes
- It is unclear if Hermione is referring to the French customs (the direction générale des douanes et droits indirects) or the British customs (Her Majesty's Customs and Excise, as it would have been at the time), or perhaps even both agencies.