Five minutes before closing time, Lilly was restocking milk when two masked men entered the shop where she worked, one with a Taser and one with a gun.
They demanded money from the till, then panicked and tasered both Lilly and her colleague Mary.
Lilly and Mary are just two of the 115 shop staff physically attacked at work every day, industry figures suggest.
The British Retail Consortium (BRC) said incidents were becoming "more violent and frightening",
Knife crime was an issue of "significant" concern, the BRC added.
The attack suffered by Lilly and Mary – whose names have been changed – is outlined in the BRC's latest report on retail crime.
The BRC said it recorded just over 42,000 violent incidents for the year to the 31 March 2018, an average of 115 every day. The respondents to its survey control 11,000 stores with sales of £103bn, equating to just under one-third of the retail market.
The total number of incidents of violence and abuse, including threatening behaviour not involving violence, rose from 350 incidents a day in the previous year to 390.
Recounting her experience, Lilly said: "There were four customers in store, and some of them shouted. The men robbers looked panicked, and thank God they ran away.
"They took £150 from the till with them. £150 – was it really worth it for them?"
In its report, the BRC said it would be wrong to assume that weapons were only used in the "higher-end incidents, or for very significant amounts".
Lilly said she was "shaken and upset" after her ordeal. "The next time I stood on the till, thoughts of what I could have done, what I should have done, ran through my head.
"I was suspicious of every man who came to the till that day, wondering could it have been them?
"But you have to get over these feelings and get on with the job, you can't think like that."
Chief executive Helen Dickinson said violence against staff remained one of the most "pressing issues" facing retailers.
"Yet once again, we have seen an increase in the overall number of incidents. No one should go to work fearing threats and abuse."
The BRC said the most worrying triggers for violence were:
Respondents said knives were the "most significant" threat, followed by syringes and "hitting implements".
"Guns were also concerningly high," the BRC added.
About 70% said the police response to retail crime was "poor or very poor".
Ms Dickinson said : "We hope this report will act as a catalyst for Police and Crime Commissioners around the country to take action.
"Retail crime should be explicitly addressed by Police and Crime Plans. Furthermore, Parliament must play its part in stemming this tide of crime by creating a specific criminal offence to protect retail employees from assault at work, as has been done for emergency workers."
The BRC's Retail Crime Survey also found that retailers lost £900m to all forms of crime over the course of the year and that crime prevention cost £1bn.
The combined total of £1.9bn is equivalent to about 20% of the estimated profits of the entire retail industry, it said.
The cost of customer theft rose by 31% compared with the previous year, to £700m.