Service due and codes

I have an 03 E500 with 69000 miles. The car runs like a dream with not a single problem noticed. I'm due for a D service according to my computer. I also have a bunch of numbers (codes) I see when I scroll through the in dash computer that list maintenance procedures. Is there an online site that could tell me what a D service is as well as what the numbers mean or can someone tell me? Can anyone tell me what a D service generally costs? It was suggested that I just do an A service. What is your opinion? I normally wouldn't be concerned and just do what it tells me but I'm a victim of the recession and unemployed since December. Things are getting tight! TIA Howard