Not for sure I just guessed 20 pounds it shouldn't be much more than that if weighs more, I sent my engine (60 pounds) and it was cheap as I said above!
you might as well print the shipping lable off line, its so much easier. Put your plastics in a box. Weigh the box! The enter all of the details into the calculate a price thing. Pick the cheapest one. You can have the postal worker pick it up at your house if your really lazy lol.
Heres the link to the calculate a price: Postage Price Calculator