Currys PC World has apologised after customers complained about being pressured into paying more when buying laptops.
Consumer group Which? said 108 customers had reported being charged up to £40 more than the advertised price.
The group said it had raised the issue with the retailer “multiple times” since 2015, but continued to hear from customers.
Currys PC World said it was “urgently re-briefing” its stores.
Customers said staff had told them computers that had already been set up were the only ones left in stock, meaning they would have to pay a previously unmentioned set-up fee.
The retailer offers a £35 “Knowhow” set-up service. Customers told Which? they were not advised it was optional.
Under UK consumer contracts law all retailers should advertise the full price of a product bought online.
A Currys PC World spokeswoman said: “We are sorry to hear that some customers have been charged for a Knowhow Laptop Set-up service on their new machine when they did not request it.
“While setting up machines in advance enables customers who want the service to benefit from it straight away, it is not something everyone needs.
“We are urgently re-briefing our stores now to remind them that, in the small number of cases where only pre-set up models are available, customers should not be charged for the service when they buy their laptop.”
Customers affected by this should email Currys PC World at email@example.com, she added.
Which? called on the firm to refund affected customers, saying it had first contacted Currys in January 2015.
Alex Neill, Which? director of home and product services, said: “This issue has been going on for more than three years without resolution and we are disappointed people are continuing to report feeling pressurised into parting with their cash.
“We want Currys to make cast-iron guarantees that it will put an end to this practice and that customers who’ve been caught out will be reimbursed.”
In 2013, hundreds of Currys customers complained they had been mis-sold extended warranties.
Currys said at the time that documents seen by BBC Wales were a record of why people cancelled a warranty and did not prove mis-selling.