brd makes a good point, but...
If UPS and FedEx are the 2 biggest courier-service companies in the US (with at least 80+% combined market-share), they have no incentive to decrease costs except as a planned duopoly to show they are "competing".
Cost drop should be minimal, as they will swallow the the value as "profit".
##I don't know if the duopoly scenario is true or not, but that is my opinion if they exist as such.
I would imagine that UPS/FEDEX, like airline carriers, play with futures to stabilize their fuel costs. As such, they are likely not reaping much in the way of immediately benefits from the recent and unexpected price drop.