"The police are in the middle of their huge summer test purchasing campaign and we have recently heard reports of retailers failing test purchases and their staff being served with fixed penalty notices," a spokesman said.
If accepted, the £80 on-the-spot fine ensures that no further legal proceedings will be taken against the individual.
A 10-week campaign to target stores across England and Wales began in May, with chosen outlets receiving three test purchasing stings over a six-week period. Should the store fail one of these, two further visits will take place over the following four weeks. Retailers will be notified if their store passes or fails a test purchase.
Earlier this month a store owner in Eccles was fined £750 and banned from selling alcohol for three months after failing three separate test purchasing operations. Khan Hussain was also ordered to pay £525 costs.
The ACS warned that failing just one test purchase could result in a licence review, where the licensing authorities have the power to modify the conditions of a licence, or even revoke it. If a store fails three test purchases it could face an immediate 48-hour closure notice for selling alcohol as well as a steep fine.
Retailers had the tools to combat underage sales, said the ACS, they just had to use them.