There are two types of Stretch goals. Those that get you new material for free, and those that unlock new add-ons.
Every Free Stretch Goal will state who will recieve that free stuff, be it for everybody or just for some pledges (For example, MECH INFANTRY PLATOON Pledge will get no free magnets, as it has no turrets). If for any reason there is no information in the list of Stretch Goals we will explain it in the different updates.
New ADDON Stretch Goals, on the other side, unlock new material that can be purchased aside from the initial pledge. In order to get this new add-ons just add the amount to your total contribution, and once the campaign ends we will send you a survey so you can state what you want to spend your money on.