Retailers focus on the store, the layout, display but sometimes other things define the visit more. Contrast what happens in Sainsbury's (can i pack your bags) and IKEA (your boot being too small) and M&S (5p a bag).
Most of us can afford 5p a bag but the problem is your start thinking in terms of bags. Do i need another plastic bag to perhaps i don't need that extra bag of Bruschetta. The notion of constraint is embedded in our heads by the shopping process, all because of 5p. M&S don't capture customer data at at an individual level so they probably don't understand how this impacts on purchase behaviour. Anyone know?
Other supermarkets suffer similarly. They offer a discount for buying 6 bottles of wine. Excellent. But they always run out of boxes, and carrying 6 bottles in plastic bags is not ideal, especially the Rioja covered in wire. So what do you do? Buy two bottles not six.