Sure why not Amaan Kulshreshtha so let me explain the reason why i have suggested you to have Mongo DB for Document Data Store and MySQL for Transnational.
As eCommerce is so big and categorized market and we have to manage "n" number of products. And MONGODB is proven document oriented NoSQL database. As this is schema less we can mange categories of product and also suitable to store a product as a documents.
When we come to MySQL already proven by industry as robust transaction support also in terms of scalability and flexibility.
EX: Lets say the product you are selling is a document which might have a structured or unstructured data or keys which can be easily stored in MONGO DB, so basically we can say as we found the need of storing some unstructured data this looks more helpful. When some is going a head to pay some amount we make sure to enter and store the key's properly so it always set very few specific mandatory and structured value & attributes.