Doesn't that only apply to cigarettes and spirits nowadays? As far as I am aware, the price is the same in each case, and the retailer pays for the intra-EU duty/VAT if it is an intra-EU flight (that's why they ask for your BP and enter the flight number; rumour has it that many retailers exaggerate the proportion of non-EU pax in order to reduce their VAT bill).

This wouldn't work for cigarettes and spirits as the duty on them is so high that no retailer would want to offer such a deal.