Originally Posted by
mnbp
There are two ways to buy C+. One is at time of ticket purchase, and the other method is from the seat map - after ticket purchase.
Since C+ is not available on your dates for the MSP-BIS segments, they can't sell the C+ through fare. However, after you purchase your main cabin ticket (PHL-BIS), you can purchase C+ from the seat map just for the segments where it is available (PHL-MSP).