WHSmith has been fined after a customer fell almost 3m (9ft 8in) through an open trapdoor on the shop floor, leaving her with serious injuries.
The 64-year-old was looking at birthday cards when she fell into the basement after staff opened the door on the trading floor, in February 2014
WHSmith pleaded guilty to health and safety breaches and has been fined £337,500.
It apologised and said changes had been put in place since the incident.
A member of staff who was standing by the open trapdoor "grabbed for the falling customer, and managed to grab hold of her, but not to stop her fall", Taunton Dean Borough Council said.
The woman was taken to hospital and "required three operations to rebuild her heel".
Since the accident, the authority said WHSmith had locked the basement and completed a risk assessment for accessing it should the need arise.
Councillor Patrick Berry, said "A customer suffered serious injuries as a result of failings in safety procedures at the Taunton WHSmith store.
"One of our top priorities is the wellbeing of people in our borough, so I hope the size of this fine serves as a warning to all businesses, that the health and safety of all who use their premises - be they customers, employees or other visitors - is paramount."
WHSmith pleaded guilty to health and safety breaches at Taunton Magistrates Court in May.
At a sentencing hearing at Taunton Crown Court on Wednesday, the company was fined £168,750 for each of the two counts of breaching health and safety laws. It was also ordered to pay £135,492.66 in costs and a £120 victim surcharge.