I am one of those who believes that MongoDB can be used for everything, this thanks to the advertising of MongoDB.
We are creating an e-commerce platform, we know that it has many relationships, but with MongoDB we can avoid some, but in the end, some relationships have to exist.
A single developer to create two native applications in Flutter, a web application with React, create the backend with multiple microservices hosted with Google Cloud Run. PostgreSQL can be heavy because it should be used with an ORM, on the contrary, with MongoDB you can avoid some relationships and avoid ORM / ODM.
We need advice from someone who has the experience and has had to choose between these two databases for an e-commerce site.
Had exactly the same question when selecting data storage for our new product. Not e-commerce though, rather interactive and content-focused HR SaaS for SME.
The key arguments for PostgreSQL
It gives you the opportunity to use relationships where you really need it and just go with key-value tables where you don't.
With Jsonb datatype you can store documents/objects/arrays as JSON then use JSON elements in queries and even indexes.
There are more tools/integrations working with PostgreSQL which you can use out of the box, e.g. Hasura
The real question here is not about the technology but rather your real needs and your data. Do you need to manage data that has core concepts and relations ? (such as a family, with parents and children) or do you need to manage a basic collection of similar data (such as blog entries)? PostgreSQL is definitely a relational database for managing entities and their relationships whereas MongoDB (I may be strongly opinionated here ;-) ) is more targeted at managing collection of entities (such as the blog entries). For an e-commerce site (with some products, products categories, user ratings and comments, prices, bundles...) I would go for PostgreSQL as it will support/guide you in creating a structured data set with all your products, organized in categories and with user ratings/comments attached to them. HTH