Hook Website To Invoice Software

Hi We have a website which is currently awaiting stock to be added to it (view here [login to view URL]). The principal of what we want is when a user clicks 'Customer Login' they are presented with a login screen where they enter their personal details that were given to them (there will be no sign up page). When the user successfully enters their login details they will then be presented by a form where they can fill out their order details and hit enter where it will add the order to the system for the client to view. I've attached some files for reference (design, work flow document). Please note that there will be NO MONEY TRANSACTIONS on this website, as its for existing / private clients they already have a way they pay for invoices with the client so all we need to do is simply be able to update the database with orders. They are currently using 'Pegasus - Opera accounting software' so would ideally like not to change platforms, but we aren't sure if it's capable of this functionality. [login to view URL] if this isn't possible we are open to recommended software they would be able to use. A website with similar functionality is [login to view URL] Please IM with any questions. Thanks