After you purchase the fire extinguisher you need to upgrade you perks to unlock "Persuasion Techniques."
This is right above "Sweets And Snacks VII," after you have purchased this perk, take your fire extinguisher and stand out in front of your store and start spraying people walking by with the fire extinguisher. You'll want to get people who are not already coming into the store or leaving it. You'll need just a person walking by to be blown into the store. Once they are in the store they will say either 1 of 2 things. One will be about how you can not convince me to purchase anything (this is not the correct response), or something along the lines of "Fine Fine, I'll buy something" (this is what you are looking for.
At the time of writing this there is not a tracker for how many people you have 'convinced' to shop in your store. Just take a day and continue pushing people inside. Once you have 10 then you are good.
The achievement will unlock after you close the store for the day after you have 'convinced' your 10th bystander to shop in your store.