Matthew Whelan, 29, from Mancot, Flintshire, had previously admitted wounding with intent but denied attempting to murder Imtiaz Ul Haq.
He changed his plea at Mold Crown Court on Monday.
His sentence was adjourned pending the outcome of the trial of a co-defendant.
Mr Haq, 58, suffered serious injuries to his throat during the incident at the Costcutters store in Queensferry on 8 December.
Whelan, who has previously admitted a charge of robbery, was remanded in custody.
Co-defendant Leslie Baines, 47, from Connah's Quay, denies robbery and is due to go on trial in May.
A man has admitted a charge of attempted murder after a shopkeeper's neck was cut during a robbery at his premises.