What you can buy is various degrees and quality of sensible defaults and behaviours that serve your general interest in privacy and security. Privacy from casual snooping or commercial tracking, security from unsophisticated attacks or even sophisticated attacks if you're wiling to also sacrifice some convenience.
These are all worth having, and your choices of product and service provider can have a significant impact on them. I know little to nothing about ProtonMail but maybe they're a better bet than many other similar services, even if they're not perfect.