If you are planning to ship yourself any resupply boxes this trail season, consider sending them UPS-Brown or FedEx. (Not USPS.) Reports from maildrop locations are that boxes being sent via the United States Postal Service including Priority Mail are not being delivered in a timely manner.
The Colorado Trail Foundation always suggests getting directly in touch with your prospective maildrop location and learn their particulars. Confirm things such as whether they are accepting packages, whether there’s a fee, and which shipping carrier(s) are best. Some of the small towns along the trail are not served daily (or at all) by the Postal Service, and we would not want you and your resupply box to miss each other along the way!