Don't know how to set this up or what kind of technology I need to add to her HTML site. She uses paypal currently to send electronic invoices but wants her clients to be able to login to a secure area where they can pay them directly on her website. Do I have to get a programmer to build something custom for this or is there something pre-built that I can use? Thanks so much!
You could have someone build you something custom if you wanted or you could use a ready made system like storefront: http://www.storefront.net/ There are many different systems out there that you could use, however they will most likely not match your existing site design, so you would then have to look at skinning the system to match.