What does a person need if he or she is not able to pay for a planned purchase in full with a check or cash?

A loan or grant

The opportunity cost is the jacket; the benefit is that she has two shirts.

Opportunity cost is the cost of the alternative forgone in choosing the other alternative. 

She chose to buy the shirts. In doing so, she gave up the chance of buying the jacket. Thus, the forgone alternative is the jacket. It is the cost of lost opportunity when she opted to buy the shirts at $25 each.