I'm looking at purchasing an 08 E550. When I spoke with the dealer, the rep told me that there was an IWG or IGW tax/fee. I didn't get a chance to ask her what it meant. What does that mean? She said it was $600.

They are probably talking about the advertising fee. Its a MB program dealers sign up for and then they add that fee to their invoice.

It's listed as IRG Participation on my invoice. $500 in central WA, although I'm told not all dealers participate.

It's $600 in the SW US. I suspect that the vast majority of advertising is done in Southern California and the rest of us have to pay for it

thanks for clearing that up. Is the IRG participation something the dealer can completely erase from the invoice or is it a mandatory charge?

The Invoice indicates what the dealer pays MBUSA for the car before any discounts and the advertising fee is on the Invoice. In other words, the dealer pays the advertising fee so it's passed on to you.