Is customer access free? Can they enter sales orders on Customer Center?

Yes

Thank you. Is it pretty common to use that?

Yes to using free customer access. No to entering the orders through that customer centre UI.

Thank you. May I know why do you discourage entering sales orders in the customer center?

Customers enter orders via the website shopping cart.

We don't really have a website. Some of our customers are okay with using a portal to enter sales orders.