Once you have a Photopost Pro license, you're free to make any code modifications you desire. Photopost Pro, "off the shelf," is already well-suited to professional photographers in many ways. For example, without any code changes, password-protected galleries can be created by the administrator for clients to review their proofs.
Various code modifications, publicly shared via photopostdev.com, this website, PayPal's developer network (if integrating with PayPal's shopping cart), or provided by commercial developers, some of whom also work for Photopost, can extend the capabilities of the software further -- for example, to allow it to serve as a basic shopping cart or more advanced client photo review site.
You're welcome to wander the Photopost Pro installation my husband and I use in our professional photography business, Moore Photography & Design
, to get a feel for what the software can be made to do.
That said, I do not offer my development or design services to others at present. Our own websites keep me busy as it is. I just wanted to chime in and provide something you could look at to see if Photopost can meet the needs of your business.
Truth be told, none of the changes I made are incredibly hard to do, though they do make upgrades more time-consuming and do require a high comfort level with database s and PHP (if not, learn to make backups so you can fearlessly experiment until things work as you desire).