I am currently learning ASP.NET Core, and have been enjoying the great integration between SQL Server 2017, ASP.Net Core, and all that entity framework provides. I am currently using the express edition, which is free.

If I start doing freelancing for small jobs in my area and use SQL Server as the database of choice for my clients shops (likely going to be small scale and <10gb), can I use SQL Server in production and not have to pay for it? (As in, continue to use my express license). I just want to double check incase i'm interpreting things incorrectly.