The best way to accomplish this is by using a graphic header. As well as simply looking more professional, a graphic header gives you full control of the practice detail space printed on receipts/invoices, in particular the ability to list additional information such as a website, email and PO Box address.
By using a graphic header the entire text output of the clinic details is replaced by your chosen graphical header. This means that you will need to design a graphic that incorporates your clinic logo, practice name (if not already part of the logo) as well as the practice address, and contact information (phone, web, email). In answer to your question, you can also incorporate your PO Box address into the graphic.
Below is an example of the way your receipt displays when the practice name and address are entered in as text:
and here is the same receipt using a graphical header, note that all text is replaced by the graphical header:
To ensure the graphic header is printed in in the highest quality across all possible document sizes in Front Desk, you must include vector graphics and not rasterised/bitmap images (e.g. JPG). To make sure that your text isn't altered all fonts must be converted to paths/outlines. When submitting graphic header requests the file must be in AI (CS3 compatible), EPS, PDF or EMF format, less than 2MB, and width/height ratio must be to 3:1. You may need to consult with a graphic designer during this process.