Can I pay social security when opening an online shop in Shanghai? How come?

If you have a business license or business license for individual industrial and commercial households, you can go to the social security center at the place of registration or business operation, bring your business license, organization code certificate and official seal, open an enterprise account, and then pay social security in the unit account. Otherwise, people who are not registered in Shanghai cannot pay as freelancers.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.