I have a domestic 2-direct-flight round trip booked for later this week. I have non-refundable (W & N) class flights. Due to illness, I might want to postpone this trip and re-book the same thing again for sometime in the next month.

Based on reading past posts--- I believe that if I call CO prior to departure and cancel, I will pay $100, and then have a credit for the price of the cancelled trip. Once I decide when I want to travel again, I can use this credit to book the new flights for myself. If the re-booked trip is less expensive than the original cancelled trip, I may get a credit / ETC for the difference. If the re-booked trip is more, I would have to pay the difference.

Is this correct? And if I decide to cancel, is the best way to handle it calling the reservations office, or can this all be done online? Thanks.

You are correct, except that you will not have to pay the $100 change fee until you actually book another ticket using the credit. The credit will be valid for one year from the original date of outbound travel.