Once or twice a year we have a customer that has a particular bird that rakes feed out of the feeder despite the 1/2" lip inside the feeder. This small sheet metal part just slips between the back side of the front edges of sides of the feeder and the actual front panel (which is actually part of the bottom and back, all one long part). Friction holds it in place, no fasteners are needed.
Another way to fix this is to cut a section of chicken wire and lay inside the feeder on top of the feed.
The lip extenders are mailed in an envelope via U.S. Mail and the shipping charge is included in the $3.62 sell price for one lip extender.