Generally heavy things are expensive to post, especially internationally, and boxes and instructions are quite heavy - their dollar value is tiny compared to a comparable weight of Lego. If you haven't turned on weight-in-cart on your options page, do so as it will help you to guesstimate postage costs - as a rough guide, if you can't figure out who the national postal service is, use you local postal service - but be aware yours (eg USPS) may be a state-subsidized postal service, so can be quite cheap compared to a for-profit postal service (eg Australia Post).

Melbourne customers can expect shipping to cut out at AUD$7.70 regardless of weight or size, so go ahead and order stupidly light (non-dense) things like sets, as I'll pack with lots of protective air. International customers get their air for free too, so can expect lots of protective air. Other Australian customers don't get free air, so I pack to minimize that.