With your priorities in line, you can create your Requirements Document. The more you can think like them, the more likely you'll create a solution that effectively caters to them. For example, you may sell a home office solution or a corporate cubicle solution. Too much information may cause a customer to leave confused. If you plan to let customers purchase more than one item at a time, then you will need to implement an online shopping cart, shopping basket, procurement list, or whatever metaphor is appropriate to your brand.
Some online stores precalculate tax and shipping expenses.
Others ask that you trust them to add regional tax and reasonable shipping expenses to the total that appears on the screen. If you will be preauthorizing payments, you must calculate tax and shipping in advance. Extensive authentication lowers your risk but decreases the ease with which customers can conduct transactions, possibly leading to lost sales.