Go to the Walmart meat section and ask for some meat boxes, they are generally heavy duty cardboard and water resistant.
Mail has been returned to the sender due to it being shipped in cardboard boxes that previously held motor oil, paint, paint thinner, chemicals, alcohol...the mail guys will not open it to inspect it and verify the contents, they just return it to the sender. (This happens so frequently that they even have an AFN commercial about the problem.)
Don't use rope, duct tape, or zip ties, just use the clear strapping tape.
Place a heavy duty yard trash bag in the box and place the items inside of that, tape it shut, then seal up the box. Sometimes mail will be palletized and sitting on the cargo ramp waiting for a flight, sometimes it rains, sometimes the guy palletizing the mail didn't do a good job and the plastic is torn or open in a few spots...and your mail gets wet.
As fas2c mentioned be generic on your customs form or it may be broken into and items may be stolen. Ship it via Priority Mail and insure it for the full value; when it is shipped like that there are tighter controls on it and greater accountability as in every time it changes hands someone HAS to sign for it. Conduct a detailed inventory and take pics, save all that for your use if anything turns up missing. I haven't heard of any mail being hand inspected just for the sake of being inspected although the majority of it does get x-rayed.