One is to a sample letter you can download and tweak, then send to the committee that approves new postage stamps. Certainly the 400th anniversary of the Mayflower's arrival is stamp-worthy. The Postal Service issued a special stamp at an earlier Mayflower anniversary and 400 years certainly calls for a stamp or even a set of stamps. Consider downloading their model letter and firing off your own missive to Washington, D. C. Be a policy influencer! Then you can brag to your friends next year about how you helped make the Mayflower quadricentennial stamp set a reality. The Postal Service issues lots of commemorative stamps and likes ideas for compelling, colorful imagery with a U.S. theme so if you don't like the ideas suggested in the letter (and I do not) feel free to swap them out for your own.
The other is to their postal artwork contest. Thinking "positive," new stamps are issued with a cachet - a fancy envelope with a complimentary design, usually postmarked at a place related to the topic. An example would be an envelope with a fancy Plymouth Rock on it (I am obviously not artistically visionary enough to win this contest), bearing one or all of the commemorative stamps you have lobbied for, and postmarked at the Plymouth or Provincetown, MA postal service on the exact date of the 400th anniversary. That would go straight into many a philatelist's collection. They need your artistic contributions, though. And teachers, assigning this as a project is a clever way to combine history and art for a curriculum double-whammy. Yes, ordinary people and ordinary artists do design the Postal Service's stamps and I believe they do pay, so when you're done with your cachet design, try for the stamp.