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 seven-wicket victory over Middlesex sealed Glamorgan's place at the top of the Southern Group and a home tie with Leicestershire in the last eight.
"It would be a great achievement, it's really important to perform well in the quarter-final," Morris said.
Glamorgan will try to secure the return of batsman David Miller for the match.
The South African left-hander, who was contracted to Glamorgan for six group games, is due to be playing for South Africa 'A' in Bloemfontein until the day before the Cardiff quarter-final on Wednesday, 23 August.
"It's going to be very tight, no question, but if David's here it's a bonus, if he's not, we still believe we've got a squad to perform well since we've won a lot of games without David," Morris told BBC Wales Sport.
Glamorgan have only previously reached Finals Day once, ending in a semi-final defeat against Leicestershire in 2004, while they lost in the quarter-finals in 2008, 2014, and 2016.
Coach Robert Croft was pleased to end the home campaign with a win in Cardiff, albeit again shortened by rain, after four wash-outs and two defeats.
"The supporters that have been here all summer, shall we call it a summer? It's been important we got a game on and showed people what we could do, we played very well," he said.

Voges has missed more than two months with a calf injury while Robson has recovered from a hamstring problem.
However, fellow batsman Nick Gubbins will not return from a hamstring problem until early September.
Bowler James Fuller, who has not played since May, will miss the rest of the 2017 season following elbow surgery.
The 27-year-old, who featured for England Lions last winter, last appeared for his county in the One-Day Cup defeat by Kent in May.
Ryan Higgins' shoulder injury picked up while fielding against Glamorgan on Friday will rule him out for three weeks.
Paul Stirling, whose calf problem has restricted him to T20 cricket, is set to be available for the Championship match at The Oval, as reigning champions Middlesex look to move clear of the two relegation places in Division One.
A man has admitted a charge of attempted murder after a shopkeeper's neck was cut during a robbery at his premises.
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Glamorgan chief executive Hugh Morris says it would be a "great achievement" for the club to make T20 Blast Finals Day for the first time since 2004.
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Adam Voges and Sam Robson have both been declared fit for Middlesex's County Championship match with Surrey, which starts on 28 August.