Carling was advertised at the show at £13.49 a case, but orders were invoiced at £13.99. Nisa blamed the rise on duty increases announced in this year's Budget.
One member contacted the C-Store office to say the lower prices were still being quoted on the website three weeks after the duty rise, and that he ordered a large quantity of stock as a result.
The retailer told C-Store: "I feel like I've been overcharged. I understand that they can't do much about printed material, but they could easily have amended the website."
Nisa's managing director for central distribution John Sharpe confirmed there had been price changes, but that this happened regularly because of the timing of the Budget.
"We communicated fully to members through a disclaimer in all documentation, including online, that these prices may be subject to change as a result of duty increases," he told C-Store.