Padded bags with the normal paper covers seem to be OK, it's just the plastic Officeworks ones which seem to have that trouble (or recycled mailers with sticky tape on them), and almost always with the classic 'lick and plonk' type stamps rather than the self-adhesive ones.
If you want cheaper still and have a concession card, make sure to get the 50 stamps for $30 they offer on a yearly basis (e.g. if you buy them now, you can't buy any more until next October). Just remember that 3 concession stamps still equals 3 regular $1 stamps (they are still regarded as being worth $1 each despite costing 60c instead), so don't go overboard with them unless the weight requires you to use 3 or 5 of them.
And of course, make sure it still fits in the letter gauge, as crinkles/folds in the mailer can actually bump up the thickness, especially if you have a habit of recycling other people's mailers from items you bought!
