09 sierra remote start prep?

Sorry if this question has been answered elsewhere, but I am new to the forum and was not able to find an answer by searching through existing posts. I have a 2009 GMC Sierra ext cab SLE and I have confirmed that I do have RPO code AP8, so I know I have the remote start prep package and that all I need to do is buy new key fobs and have the dealer flash the computer module to enable the remote start, at least I think it's that simple :confused:. However one local dealer is telling me I have to fork over $200 for part number 17801236 and an additional $80 for install. I saw that part number 15913421 ($35 each) might work with my truck (http://www.automotiveforums.com/vbulletin/#) and I'm wondering why dealer is telling me I have to buy the more expensive ones. Are 17801236 and 15913421 interchaneable and will 15913421 work with my truck? The last eight of my VIN is 9Z231628 and I can provide full VIN if needed but dealer only asked for last 8 so I was assuming (I know I shouldn't assume anything) that is all that's needed to potentially help. Thanks in advance for any assistance.