Large boxes (seats, luggage, fairing parts, etc...): USPS, FedEx and UPS are getting insanely expensive these days. A friend of mine turned me on to
https://www.shipbikes.com/HowMuch.aspx . It's actually a company that resells FedEx shipping and is intended for bicycles and bicycle parts but to FedEx it's just a box so what you're shipping does not matter. Their prices are usually 30-35% less what FedEx charges retail and you get exactly the same service. And if you're a long ways from a FedEx location it's only $5.25 more to have them pick up at your door.
Smaller boxes: You'll need to have a Paypal account but if you use Paypal shipping you can get a better than retail rate for USPS shipping if you use this link:
https://www.paypal.com/shiplabel/create/Packages less than 16 oz and no dimension over 12" can be shipped First Class Parcel Mail pretty cheaply. The prices are also discounted for Priority Mail.
