In general, the cost to mail your backers their rewards (postage, envelopes, etc.) should be taken into account when you set your goal and price your rewards. Also keep in mind that any funds pledged towards shipping costs will count towards your project's goal.
Unless you specify otherwise, backers can pledge to your project from anywhere in the world. It’s always fun to have a backer from across the globe, but it can also mean paying for international postage to send rewards overseas.
As you build your rewards, you’ll have the option to specify global shipping costs, and shipping to certain countries. During the course of your campaign, if you find that you'd like to offer shipping options to more countries, you're welcome to add more shipping specifications or create new reward tiers. For more information on setting shipping costs for your backers, read on here.