Another tactic that could be used is to add multiple actions, in an "doing A cancels B" setup. So, you'd add
Buy DVD blanks @Mall A (@Mall A means Mall A is the context, not part of the action title)
Note: cancel <link1>
Buy DVD blanks @Mall C
Note: cancel <link2>
<link1> points at the Buy DVD blanks @Mall C action
<link2> points at the Buy DVD blanks @Mall A action
You can create the links by selecting an action and doing Edit->Copy as Link, then pasting into the notes field of the other action. The result will look like
omnifocus:///task/n1Wc6vj_9Wq
Now you go off on your errands, happen to end up at Mall A, score your DVD blanks, and before you tick off that action as completed, you click on the link in the notes field which will pop up a new OF window pointing right at the linked action, which you can delete as it is no longer needed. Close that window and tick off the action you just completed.
For a simple task like this, you could probably just skip the link business altogether, but it makes for a clear illustration of what you could do in a more complex scenario where you might want to coordinate actions between different projects.