Do use comb binding or similar. It makes your portfolio compact so it's easier and cheaper to send. Also it's flexible so while being secure it still allows you to make last minute changes.
Do not use ring binders or lever arch files. They are less secure and more expensive to send by post.
Do not use plastic pockets for your pages. They aren't easy to read and do not photocopy well.
Do consider submitting electronically following these guidelines.