After you add on the cost of fuel – I doubt there will be any savings in your shopping at St. Omer Vs Calais, and you certainly won’t find such a wide range of wines.
I regularly take a booze-cruise across to Calais; although there are many Brits shopping in Carrefore and Auchan – there are also many locals shopping there too. The Calais Auchan store is actually in Coquelles just outside Calais, but it is well sign posted (and worth a visit).
It is also worth fuelling the car in France with unleaded at around £1.20/litre.