The Post Office is to take on rivals including Argos, National Rail and Transport for London in the race to attract click-and-collect shoppers as the 350-year-old service's strives to enter the digital age.

A trial number of hi-tech post offices – the first of which could be open by Christmas – will feature code-operated lockers that people can use to collect goods bought online. Other facilities will include machines for posting parcels or picking up fishing licences, and booths where customers can consult a mortgage adviser by video.

Open-plan service desks will replace the old counters with staff behind secure glass; instead of posters taped to walls, interactive digital screens will offer information on the latest services.

The hope is that slicker, more convenient post offices will attract a greater number of small business owners and ordinary shoppers, and help boost sales of financial services such as current accounts, insurance and mortgages.

"We know people like and trust a post office but it has absolutely got to change," says Pete Markey, newly appointed chief marketing officer at the Post Office, who is leading the modernisation programme. "There is so much more to go for if you can get better at doing what people best know you for."

Within the next year, about six post offices will be revamped with the new sleek design, inspired by the Apple Store, and automated lockers similar to those used in digital outlets of Argos – where the familiar tiny pens, order slips and paper catalogues have given way to tablet computers. More outlets will quickly follow if the trial is a success.

This is the latest stage of a £2bn modernisation programme that is already seeing 10 post offices a day get a refresh, funded by government cash intended to secure the future of the service after its split from Royal Mail two years ago. So far nearly 3,000 branches have been modernised and a further 5,000 will get a makeover as the service attempts to wean itself off government support.

Unions welcomed the modernisation programme but are concerned it may lead to further cuts after more than 1,000 employees of Crown Post Offices lost their jobs in recent years.

Billy Hayes, general secretary of the Communication Workers Union, says: "Modernisation has resulted in the closure or franchise of 75 Crown Post Offices across the country – a fifth of the nationwide network. If the Post Office is serious about modernising, improving its services and ensuring the current network remains, it must focus on attracting quality subpostmasters instead of offering them compensation to leave the business. These subpostmasters need to be paid a decent rate to run the service, and to ensure their staff are well-trained and provide first-class customer service."

But Markey insists automation is not about cutting staff numbers. "It is more around what they do," he says. "I don't see us moving to a stage where there would be completely unmanned or unmonitored post offices. It's about making things more dynamic and interesting for staff – across a self-service kiosk rather than behind a counter all day. At the heart of the business are those characters who work behind the counter and their relationships with customers. It's about choice. You can choose the way you want to interact."

The Post Office already offers click-and-collect for online purchases in partnership with Royal Mail: shoppers can opt to pick up parcels at their local branch. But the hope is that automated lockers would make that service more accessible. The Post Office has extended opening hours to include Sundays at many branches, and Markey says these retailers are enjoying a 10% uplift in sales as a result. He says the lockers will be trialled both inside and outside branches, where they could potentially be available 24 hours a day.

With click-and-collect one of the few areas of rapid sales growth for retailers, an ever-growing number of service providers are aiming to cash in. Logistics company ByBox is helping supermarket Waitrose install refrigerated collection lockers at Gatwick airport; Argos shoppers will be able to pick up eBay parcels from 650 stores by the end of this year; and London Underground and Network Rail have both announced plans to turn stations into pickup points for a host of retailers. Local independent shops are also getting in on the act via the CollectPlus network.

Markey has been trialling the Post Office's automated services with groups of customers and staff at a "lab" store near Elephant and Castle in south London for about a month.

"Innovation and experimentation are really important in this market," he says. "We are entering uncharted space, testing and trying things, and we want to be fleet of foot. You have got to let people play with things and see if they work."

Some 500 self-service kiosks have already been installed in 180 post offices: these allow customers to pay bills and buy stamps for parcels. The new machines will make it possible for a transaction to begin at home or on a tablet or smartphone, so that processes can be quickly completed in the branch.

Online services have to be the way forward. "Much of the relationship with the Post Office will exist outside of the post office in future," says Markey.