You will need MySQL because you will frequently perform transactions in an eCommerce app. MongoDB doesn't support "multi-document" transactions. Although you could do a two phase commit to overcome this problem, I don't think this is the right approach. Instead you should go with a relational DB like MySQL to support transactions/rollbacks etc.
So, the bottom line is that MySQL is definitely a better choice for financial transactions (which is your case). You could also have both MySQL and MongoDB. For instance, MongoDB will work well for certain parts of your web app such as product catalogs. And you can use MySQL for handling transactions, storing customer details, order tracking etc.