Some of the decision depends on what you want to use your Custom package for and how big your custom package is. The 50% km rate or the cost of shipping the trailer - either will come out of that package.
Scenario 1 - you ship the trailer, the bill is paid by IRP, the trailer arrives at your door, money is spent from your Custom package
Scenario 2 - you tow the trailer, get an extra 25 cents/km (roughly, depending on the province), and it probably won't cost you that in extra gas, so you have a bit of money in your pocket for your trouble, also from your Custom package.
So depends on how much trouble it is to tow your trailer, how big your Custom package is, how many kms you're moving, and so on. Do a little comparison and see which scenario works better for you.