It is a code emailed to you from DriveThruRPG that allows you to purchase the POD copy at a reduced price (rough estimate in the Goal section) plus the actual postage cost.
Example: If you pledge for the Delve POD, you pay $25 now and ~$12.18 plus postge for a Hardback copy when it is finished. A total of ~$37.18 plus postage versus the MSRP of $45 plus postage.
So yes, you end up paying "twice" but the total amount is less than the retail cost and the shipping cost is based on the actual weight, distance, and rates versus an estimate cost now. Plus, since DriveThruRPG works with printers in Europe as well as the USA, it would ship out from the most local area, making the postage more reasonable.