This is why Steam downloads should never be considered as a reputable source. The fact that they expect you to purchase the first SH4 download for $9.99....an unforeseen version 1.3 (which is a terrible version of the game). Then offer the U-Boat Missions Add-on (version 1.5) at an additional $9.99 cost is just plain thievery.
Buy a DVD Disk "Gold Edition" v1.5 from someplace like
iNetVideo for less than the Steam price and thumb your nose at a download.