the best way to ship this painting is to build a lightwieght crate from 1 x 4s and masonite panels,.....since you are using a 36 x 48 canvas, you'll need to plan the crate from the length of a sheet of masonite with is 48 inches, use the thinner form of masonite or 1/8 inch to be the lightest. your 1 x 4 frame will be cut as follows,......to ohuse the 36 inch side, cut two 1 x 4s to the length of 38inches, then cut two 1 x 4s 51&1/2, this will give you an inch on all sides to put packing into.........stand up the two 38 inch pieces and lay one of the 51 1/2 piece boards atop the 38 inchers, nail these in place, then do the same to the other side, your interior wioll be 38 x 50 inches. this gives you the one inch border on all interior sides. Next, using the 1/8 inch masonite, cut the outside measurments of 39& 1/2 inches by 51& 1/2 out of the masonite, this is your front, cut another one exact;y the same and then attach one side to the crate frame you just built, now you're ready to pack it. place the crate with the one side finished onto a table with the open side up, place you canvas inside, note the area you have left around the canvas, now pull the canvas back out and pack the back side of the canvas with large bubble wrap until it fills the area that's on the inside of the canvas frame to the depth of the canvas, tape it in place with packing tape and layt it backl in the crate, now fill the open edges of the crate with stri[s of cardboard until the canvas sits tight in the crate, now fill the rest of the empty part of the canvas in the crate with bubble wrap making saure that your bubble wrap doesnt push the canvas into some odd shpe beneathe the bubble wrap,..next, your lid will be the same as the back of the crate or 51&1/2 x 39&1/2 from the masonite, screw it in place and take it to yopur local shipper,......now, using this format, note:,.If you ship it this way, your insurance wont cover it if it gets ruined in shipping, the best way to ship is to pay a shipper to pack it and ship it, that way, if iot gets screwed up?......they gotta pay.....hope this helps.