I am a little confused by your question, but I think I understand what you mean. The original statement is in the form If x, then y. (if treated discourteously then customers shop elsewhere) If you take the contrapositive to the original statement (if not y, then not x), it reads-- If a customer does NOT shop elsewhere, then they are not treated discourtesouly.
The second statement, what shopwell wants is immaterial. What they will do is important. This is the equivalent of saying ~x (not x). This condition tells you nothing other than you have a necessary condtion to keep customers. It is not sufficient to keep customers, but is required if they stay.
Look at it like this, if they do not treat the customers discourteously, but they are forget to stock up and run out of all items, they will lose custoemrs anyway. Or another way is if you bake a cake, you have eggs. But having eggs is not enough for a cake, you also need sugar, water, etc.
I hope that clears it up a bit. If not let me know and I;ll try to explain a little more