In the United States v. Van Leeuwen case of 1969, the Supreme Court ruled that a delay in mail delivery caused by customs officials did not violate Fourth Amendment rights against unreasonable searches and seizures. The defendant, Jacobus Van Leeuwen, had mailed two packages containing gold coins to Canada from Washington state but they were detained by customs for further investigation due to suspicion of illegal activity. During this time, law enforcement obtained a search warrant and found evidence leading to his conviction for smuggling gold out of the country without proper documentation. On appeal, Van Leeuwen argued that holding his packages constituted an unlawful seizure under the Fourth Amendment. However, the Supreme Court disagreed stating that there was reasonable cause for detention given their suspicious nature and thus no constitutional violation occurred.

In the dissenting opinion for United States v. Van Leeuwen, Justice Douglas argued that holding a mailed package for 29 hours without obtaining a warrant constituted an unreasonable seizure under the Fourth Amendment. He contended that once postal inspectors had enough information to suspect illegal activity, they should have promptly sought a search warrant instead of delaying and detaining the parcel further. The majority's decision allowed law enforcement too much discretion in determining how long they could hold mail without violating constitutional rights, according to Justice Douglas. This lack of clear limits potentially threatened citizens' privacy rights and protections against unwarranted government intrusion.