Dutch customs officials discovered a parcel containing two handguns, ammunition, herbal cannabis and GHB, also known as liquid ecstasy, addressed to a location on Merseyside.
The seizure was referred to the National Crime Agency.
Officers raided two addresses in St Helens earlier, arresting a 22-year-old man and a 20-year-old woman.
A couple have been arrested on suspicion of allegedly importing firearms and drugs into the UK.