Just another data point for domestic flights in Colombia on XS fare. Was able to enter my TK M&S Elite number when making the reservation online (paid with cc) on AV website. During OLCI I was able to add 1 pcs checked luggage (max 15 kilo) for free. At the baggage drop counters in the airport no issue. It saved me from paying around 50-60,000 COP for a checked bag (if you buy it online in advance; it is almost 100,000 COP at the airport)

Somewhere between check-in and boarding at the gate, my seat was changed from 26A to 4A (first row behind the Premium Economy). When my BP was scanned at the gate, gate agent manually wrote "4A" on my BP with a pen (no upgrade but a still nice change of seat to the front)

Only issue was that the flight was delayed almost an hour (arrived at destination with 50min delay). Sat on the aircraft for ages awaiting finalization of "paper works" before being able to leave the gate and depart.