[Simple]

Is your wife's car made in "March 11, 2012" ?? As I mentioned, it is 4 or 5 numbers.. the first four or five numbers..
Hence her car's RPO is :12907...

And no, you don't type the RPO when it asks for the VIN, it should read the VIN automatically, the VIN is 17 Character/Digit number... But you can also type it if you wish.. If you click on the icon (little car drawing with "?"), it should read the VIN and Identify the care make and model..

RPO 9550 = January 1, 2003 and RPO 9915 = January 1, 2004 (9915-9550=365 days/year). You can work forward and backward now..

Lets take your wife's car, from Jan 1 2003 (9550) to Mar 11 2012 (xxxxx) is 9 years + 71 days (approx considering leap), which = 9550 + (9*365+71) = 9550 + 3356 = 12906... So the (xxxxx) RPO for 11 March 2012 is:12906.

Now, considering that Citroen makes changes to a model once every few years, any RPO number that represent that configuration model should work. So, if we assume that model was not changed in all of 2012, then any number between 12837 (1.1.2012) and 13202 (31.12.2012) should work.

For your dad's car, If we assume it was made mid 2005, then Based on the above, (9550 + (365+365+182))= 10462 should be safe enough RPO, you could try 10462 +/- 90 ..