The online parcel srvices work on the basis of incremental revenue. If you have set up a national distribution network of depots vans & trunking operations, then in logic it has a given capacity - say for the sake of argument, 3 million units/24 hours. If in July when the market is relatively dead you're running at 1.8 million units, then it makes sense to take on extra revenue where you can - after all, you have the fixed cost of the network to consider.
I doubt you would get the same rate in December.