Hi! You can absolutely use a P.O. Box for your required mailing address. All you need to do is locate your Settings and Billing in the upper right hand of your account and then click the 'Sender info' tab in your Account Settings. I am attaching a screenshot of what that looks like below. Here you can click enter in the new address and click Save. You are good to go from there!
I hope this helps and let me know if there is anything else I can help with!