Firstly you need to create a shop account with your correct details. Then simply click 'Buy Item' on the item's detail page and follow the on-screen instructions.
Immediately after you complete the purchase, go to your User Interface, then click on 'My Orders'. There you will find all your completed purchases, and simply click Download to receive the schematic directly to your computer.
When you purchase an item, you are actually buying a licence to use it. You are free to upload it on to your server, adapt it, or just use it for your own personal Minecraft world. You are not however allowed to use it for commercial purposes (for instance re-selling) or distribute other copies. Please check our terms of service page for full details.
A schematic file is simply a type of file that can store a Minecraft build. You can load this build into your world by using WorldEdit or MCEdit (both of which have an upload schematic function)
No, there is no fee or charge for you. The prices shown in the shop contain the license, "shipping", taxes and paypal fees.