Logistics companies use a fuel surcharge. It's a percentage of the rate (before the surcharge) that is added onto the total to account for fuel costs. The bigger the player, the more pull their surcharge has, regardless if they're a carrier or a customer. Some companies release a weekly fuel surcharge while others release a monthly one.
Creates situations where a massive company like Wal mart or Loblaws will tell their carriers this is the fuel surcharge we're going to pay and that's that, while smaller companies have to suck it up and pay whatever the carrier decides their fuel surcharge is. There is a Fuel Surcharge released by the FCA that most carriers use as their benchmark.
http://www.ntscanada.com/CFS.asp
I think when I worked at Loblaws the new fuel surcharge was done monthly.