Expediting a letter to a post office box can be done through the courier. The shipper has different shipping options available, the fastest shipping option is a same day or overnight service. Most big name couriers offer overnight shipping anywhere in the world. This is by far the fastest way to expedite sending a letter to a post office box. International overnight service can be expensive and cost up to $100 depending on the destination and origin of the letter.
Pack the letter into the smallest envelope available. The smaller the envelope and the lighter it is, the less it will cost. Include relevant shipping information such as the recipient's telephone number. UPS requires the telephone number of the recipient at the PO box. Include an address correction change or alternate address.
Compare services from the major couriers. USPS, Fedex, UPS and DHL are the most popular options in North America. Check their overnight service. The pricing and options will vary by service and provider. Most services offer a same day or overnight shipping option. The rates will also vary on the time of deliver. For example, a delivery in the morning will be more expensive than a delivery in the afternoon.
Drop off the letter at a shipping drop off center. Drop off the letter as soon as possible at a drop off location for the shipper. If the office is closed for the day, find out when it opens and be there first thing in the morning to ensure the fastest transit time.