When the operator takes the call, they have to check whether the customer exists or not first.
I could be wrong, but normally a lookup by postal code in the first instance brings back a list of possible matches.
Failing to find the customer from that list would then invoke the operator creating a new account.
I'd have thought that would be the first task.
Finding an existing customer would involve identifying their unique key, which you use to then lookup any previous orders - which seems to be the thrust of your question.
Clearly you skip that step when creating a new customer.
Depending on your situation Ajax would be ideal for this kind of demanding GUI lookup work.
But to start off, create 2 traditional work streams, existing customer and new customer, use multiple pages for each.
Once those work, investigate using Ajax to reduce those pages, and then work out how to share form elements and pop-up and hide divs on the page depending on the use-case, and try and merge those 2 work streams together - hopefully into a single user-friendly page.