If it’s available in your area, please check out FedEx Ground rates for your large and heavy boxes. Their prices are very competitive and often less expensive than parcel post.
Also, in case you are not familiar with them, the U.S. Post office has three sizes of Priority Mail Flat Rate Boxes available free at their website. The large size is 12 x 12 x 3.5, medium is 11 7/8 x 3 3/8 x 13 5/8, and small is 5 3/8 x 8 5/8 x 1 5/8. You can ship these boxes anywhere in the country, regardless of weight for one price. Click here to see rates.
These boxes come in handy, particularly to ship donations to the reservation that are small but heavy, such as school supplies and toiletries!