The United States Postal Service (USPS) is the best way to mail important documents. It is reliable and has a long history of delivering mail safely and on time. USPS also offers a variety of shipping options, so you can choose the service that best meets your needs.

If you need to send documents quickly, Priority Mail Express is the fastest shipping service offered by the USPS. Priority Mail Express guarantees overnight delivery to most locations in the United States.

If you are sending documents that do not need to be delivered overnight, First-Class Mail is a good option. First-Class Mail typically arrives within 1-3 days. Keep in mind that First-Class Mail has weight and size limits, so large or heavy documents may need to be sent via Priority Mail.

For very important documents that you want to track, consider sending them via Registered Mail. Registered Mail is the most secure shipping option offered by the USPS, and it allows you to track your mail as it moves through the postal system.