Yes. This is a common question ask daily. You can send Certified Mail to a P.O. Box in the United States. USPS Certified Mail letters are addressed and delivered to the PO Box with no changes in mailers process.   In other words you just address the letter using the PO Box instead of the street delivery address.
 
When the USPS letter carrier delivers the letter the process that documents when the letter was received  at the delivery address is a little bit different.  A delivery notification card is placed inside the PO Box instructing the recipient they have a Certified Mail waiting for pick up from the Post Office window clerk.  Because USPS requires someone to sign for each Certified Mail letter the delivery notification card must be presented to the window clerk at the USPS Post Office. Then the recipient is presented the delivery confirmation Signature card and the date and time of the delivery confirmation are electronically captured and recorded by USPS.  This is how the United States Postal Service handles delivery when you send Certified Mail letters to a PO Box.

You can save time and money by sending USPS Certified Mail by using Electronic Return Receipts a part of Certified Mail Envelopes and Send Certified Mail. These envelopes have been approved by USPS and do not use any stickers or forms.  This makes sending USPS Certified Mail with Electronic Return Receipt fast and easy.  The service includes proof of mailing, in-route letter tracking  and final proof of delivery from USPS even when you send a letter to a P.O. Box.  Old fashioned Green Card are expensive, get lost and are hard to manage. You can address your letter then print Certified Mail tracking numbers, bar codes and more all online and skip the trip to the Post Office!  No software to buy, no monthly costs or stickers or forms to fill out.

You can try the service free by signing up for the free trial.  You will still need to add postage ($2.80 + $0.44), but you can save the $2.30 green card fee and see how the Electronic Return Receipt works.  They provide the web based software to address, track and report each mailing. You have access 24/7 and they file all the Electronic Return Receipts for 7 years automatically.  USPS has approved Certified Mail Envelopes so the process is fast and easy to use.  No monthly fees or hidden costs.  All you need to do is add postage.  

• To evaluate this service you can sign up for the free trial below:  Free Trial Sign-Up 
  http://www.certified-mail-envelopes.com/free-trial/
• Follow this link to learn more about the Electronic Return Receipt – ERR.
  http://www.certified-mail-envelopes.com/electronic-return-receipt-for-usps-certified-mail/

Call us to learn more about automating USPS Certified Mail. We have Enterprise Solutions for mail houses and large volume mailers.