Fearless John Lugg, 75, was browsing through the choices when Ryan Gent burst into his local newsagents and seized terrified assistant Jacqueline Collins.
As the drunken 21-year-old held a big breadknife to her throat and demanded cigarettes, she cried out for help.
Mr Lugg – who had been hidden from view – ignored his own safety as he went to her rescue.
He grabbed Gent with the aid of fellow customer, workman Michael May.
The two men pinned him down and held him until police arrived at the Nestor”s News store in Exeter, Devon.
Gent is now starting a four-year prison sentence after admitting robbery.
The attack happened in January but Mr Lugg, a retired trucker, has spoken about it after picking up a bravery award from the police during a ceremony this week (Tue).
He said he had not thought about the dangers of confronting the robber.
He added: “I was reading the birthday cards when I heard the lady shout “help, Mr Lugg” twice.
“He couldn”t see me and as I came out I saw him holding her throat with one hand a big bread knife with the other.
“I grabbed his arm – he pushed Jackie and she fell to the floor and cut her hand.
“I kept hold of him and when we got outside the shop we got the knife off him and that was it.”
Judge Cottle praised the courage of Mr Lugg and Mr May, who risked their own safety to wrestle the knife off Gent and drag him to the ground.
Mr Lugg was given a Chief Superintendent”s commendation for “selflessness, courage and bravery”.
After picking up the award, he jokingly admitted it was “probably the stupidest thing I have ever done”.
But he added that when someone calls you for help by name it is hard not to come to their aid.
Mr Lugg said: “If there are other people around, maybe you can walk away but when you are the only one and someone calls for you it is different.
“I was completely surprised when the police asked me to come and get the award – I thought there would be other members of the public not just me.”
Mr Lugg added that he got his wife”s birthday card for free as a gesture of thanks from the shop.
And after his act of bravery he remembered to pick up a bouquet of flowers for his wife, who was 80 the following day.