I sometimes use the dark yellow A4 envelopes. Books that size can go as letter rate for up to $3.

The white ones are see through so I dont use them. This is makes the postage cheaper than buying a tough envelope, but I only do it for cheap books as well.

Sometimes cardboard can put it above the 2cm. I get books of Fishpond, but find if I reuse them, they are only good for books that would go as a $6.60 parcel rate