+1 for patience and non-confrontation when dealing with Weber CS. There are a very few folks in my experience who are difficult, and a ton who are really, really helpful. So when I catch one who is grumpy or having a bad day, I sort of shift my tactic to 'research' and explain that I'm trying to get an order together for later (to save on the $7 flat shipping rate) and just collect part numbers. Then I call back later and have always had an easy time of it. Especially when I call back on the weekend and/or late at night, when I'm convinced the more senior people are on duty.
Couple of things that have worked well for me:
1. I always register the grill I'm working on, even and especially if it's not one I'll keep. I don't do any 'flipping' (yet), but I do restore grills for friends, and having so many grills registered helps me to find a more substantial reason for needing a part from a certain vintage, etc.. It also helps avoid lying if you are opposed to such a thing.
2. I don't argue about stuff. If someone starts shoveling BS at me, I just make plans to call back later, even if it's in just a few minutes. Or even better, call back late at night when the folks know the ropes more and don't need to constantly put you on hold to 'check with my manager'.
3. Keep notes if you can. I know this is in the realm of major geekery, but I try to keep a notebook with parts numbers in it for future reference. Not to lord over the CS people, but just to double-check and for those times when they can't find something. I'm sure there are some holes in their catalogs, and having the right number will often get you what you need quick. I was recently looking for some fiber washers and the CS person was coming up with nothing. I literally went out to the shed, grabbed the parts list from my Dad's still-in-the-box 1977 SJ, read off the number, and *bingo*, I was all set.