At this point I'm not going to buy Sunset Invasion, mostly because ingame store seems to be giving me constant errors. SI isn't on the DLC list. If I go to it's separate page (by using advertisement link on CKII main menu), then it sometimes fails to load the page and when I finally get SI into my cart it refuses to accept any sort of payment* and gives error which says that "try again later or contact support". Steam hasn't done this with any other games or DLC which I have recently purchased, so it seems that just Sunset Invasion hates me.
*I have actually only tried PayPal and credit card, but it seems to be giving error before actually trying to charge money (i. e. before PayPal log in) so I doubt the method matters much.