Your online store example depends very much on the business you are developing it for. If it is a one person company that drop ships mouse mats, of course you should start with the bare minimum and build from there.

If you are working for a multinational that already sells around the world, prides itself on its reputation for customer service, always makes a huge number of extra sales on Black Friday, and is a frequent target for sophisticated hackers, you might as well take those things into account in your initial design.

Waterfall is great for the requirements that you already know with reasonable certainty. Agile is better for potential future requirements.