Know-How Exchange : Student Questions

I learned in my international business classes about purchasing power parity and how it's becoming more known with the weakening dollar. My question: when a US business sells overseas, why don't we get to take advantage of the difference between the weak dollar last week and the even weaker dollar this week. I've heard of this wiring fee for currency exchange; does it take away all potential for making money off of the weakening dollar? Can a US company use purchasing power parity to make money off of international transactions? Thanks!