is commission a separate module? or does something comes with out of box as well?

Yes it is a separate module you need to buy. It is one of the older Netsuite modules so it only works for specific use cases. Be sure to get a demo before you buy.

thanks, we only have 1 person that will get commission, so not sure if it's worth it to buy a whole separate module

i wouldnt think so. I have never got it to work for any clients. they always have something in their plan that means they cant use it.

I think we can just put on the sales order header if he got that sale, and then just do the math for percentage of the total sale

Common practice is to build a report or saved search along the lines of what you said for simple cases