Hi Mahtab and Inaki,
I was dealing with the same question and although I haven't found a 'clean' solution, I have found a workaround.
A client wants to sell a course and offer two payment options. As only one option per course is possible, I made two courses, each with one payment option. I created a third course which was a metacourse that takes enrolments from the first two (paid) courses.
Perhaps you're still looking for a solution, I hope this helps.