I tried to add fields in these following locations:
Studio > Invoices > Fields
Studio > Line Items > Fields
Studio > Products > Fields
Then I did repair and rebuild (also tried to rebuild relationship), but the added fields do not appear inside Invoices, nor when I try to create a new product.
Line Items is a special module. Fields of the module should not work after added in Studio. But Invoices and Products should support Studio without problem.