Cheap – parcel monkey, but be wary of any “quick quotes” that don’t ask for the dimensions.
Easy – take it to the post office where a chap or lady who’s job it is to know such things and who will probably have been doing it since the dawn of time will take it off you, make sure the paperwork is legit, weight it and send it on it’s way after offering you a few options in regards to speed / cost.
I don’t know if it’s my age or a change of thinking, but after years of looking for the cheapest, I’ve grown to value service and ease of use. The Post Office, getting bike fixed at LBS, car fixed at dealer, shops rather than online – it’s so easy, walk in “hello, I would like something that does this please” “no problem, you want one of these” “thank you, I do”. I just have to get over the slight sting that is knowing that I probably could save a few quid buying from some faceless box shifter, but I usually get the wrong thing anyway.