To be frank, describing Amazon as an "online store" suggests a gross simplification of their business model. Amazon's business is built on traditional pillars of retailing: customer service, B2B relations and owning the best real-estate.

No competitor is going to get better deals with UPS, Fedex and USPS. Partially because Amazon has located warehouses such that they help negotiate better shipping contracts and thus provide better customer service.

It has dumped everything that could be a profit into expansion for twenty years. A newcomer would be hard pressed to sell me a Kindle, a movie download, a used book and bearings for my clothes dryer all on the same order at an attractive price and with an aggressive delivery schedule.

If your idea might have legs, some VC will back it.

Good luck.