What are the most important tasks for your product's audience in order to achieve their goals? Sign up and Sign in may not be needed at first. Customers could order without signing in to the shop with the drawback that next time they'd have to type in all their data again.
On the other hand finding the product seems to be a must.
After you know the must have tasks, what are the tasks from these that needs IT support the most? Maybe ordering could be done on the phone first. Maybe not. It depends on your customer's business.
You'll pick the most important from the proposed solutions with your customer and schedule them into the first release.