LoL! At the moment (30.9.21) you pay 20.- Fr. at Digitec for a Free-to-Play. Because the offer is cheaper without an additional add-on! xD #fail

Most likely, "promo" means a promotion code or a bundle of Warframe. I strongly assume that the "Sister of Parvos riptide pack" is meant. The update came out at this time c.a., so it would make sense.