When you decide to pack in an extra pair of shoes into your luggage, the opportunity cost is that you might be able to bring a few extra shirts. Opportunity cost, first developed by John Stuart Mill, is the next best alternative that you have after you made a decision between several mutually exclusive choices. By analyzing the opportunity cost of your decisions, you can assess the true cost of your actions. Flying with a lighter bag allows you to fit in more stuff. If the courier charges more for each additional package, it would be better to find the biggest box and pack everything in.
The opportunity cost of your decisions aren't restricted to monetary or financial cost. Travelling with a more secure bag gives you a peace of mind. Not packing in prohibited items prevent you from being stopped at the immigration checkpoint.
So the next time you are travelling, try to think of the opportunity cost of every item you decide to bring with you. It might not be a good idea to bring 6 pair of shoes with you.