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.

The Welsh government says the idea is "open for discussion" but cites the potential cost.
It is part of plans to reform Wales's eight Community Health Councils (CHCs).
The health minister wants to make membership more diverse but the Conservatives said she has spread confusion.
There is currently one community council to shadow each local health board, except in Powys which has two.
A 12-member Board of Community Health Councils was also set up in 2004 to advise CHCs and represent their views nationally.
Health Minister Lesley Griffiths commissioned a report by Professor Marcus Longley from the University of Glamorgan's Welsh Institute for Health and Social Care.
It recommends the next chair of the board should be appointed through the public appointments process for a four year term and suggests giving the chair the title "patients commissioner".
There are already children's and older people's commissioners appointed in Wales.
"This will place them alongside other key figures in Welsh public life whose role it is to 'speak truth to power' on behalf of important sections of the community whose interests might otherwise be neglected," it says.
In a consultation document published on Wednesday, the Welsh government says changes could give the board a "stronger voice".
"Whether his or her role extends to that of a Patient's Commissioner is open for discussion, not least over the cost, role and the need to change primary legislation," it says
The consultation says Powys should be served by one CHC, but that its work should be divided between separate committees for the north and south of the county.
It also calls on the Board of CHCs to produce a report by June next year suggesting ways to increase the diversity of councils.
Ms Griffiths said: "As local health boards engage with communities on proposals for reconfiguration of health services, it is more important than ever that a broad range of people are involved in the work of CHCs.
"I believe CHCs need to look to involve groups such as the unemployed, mothers with young children, and people with full time jobs.
"They should also look at different ways of working so that people who find it difficult to attend meetings can participate."
The consultation is open until 14 January.
Earlier in the year Ms Griffiths was accused by the Welsh Conservatives of trying to "gagging" CHCs in a speech which said they "cannot be parochial".
In its consultation, the Welsh government says the Board of CHCs should have the ability to "direct individual community health councils".
For the Conservatives, party health spokesman Darren Millar AM said CHCs should be free to voice the views of local people who are concerned about plans to reconfigure hospital services.
"I urge as many people as possible to take part in this consultation in order to make sure CHCs remain a pivotal part of healthcare in Wales," he said.
A couple have been arrested on suspicion of allegedly importing firearms and drugs into the UK.
[NEXT_CONCEPT]
A patients' commissioner should be created as part of a shake-up of health watchdogs, a report by health economists suggests.