Why exactly does the Department of Dead need travel agents to work with the recently dead? Think about it - if the package you qualify for depends on how good of a life you led, then why do they need sales agents to theoretically bargain with the recently dead over their packages with? The DOD could just assign the dead their packages based on how good of a life they led; if so, then not only would there not be any actual sales involved, then there wouldn't be a hierarchy to the salesmen, either. If that was the case, then would that mean that skilled salesmen can sell clients better packages than their deeds in life qualified them for? Can the dead theoretically choose cheaper packages over more expensive packages? If so, what "currency" if you will are they making the exchange with, and what would they spend it on later if they chose to save it?
The impression I had of it was that they were not sales agents bargaining over it, they were distributing the rewards one had earned. A bureaucracy, not a shop. Manny didn't try to sell the guy who got the walking stick a better package, he just said 'this is what you get'. Don't you think the one who traveled in a packing crate full of foam would have gotten a better deal if he could? Although with the 'agents' getting points toward their own afterlife with the quality of their clients, it does seem like there's something like sales going on. Probably best not to think about it too much.
The currency they spend is money they're buried with - part of the tradition that the game is based on (Mexican and Aztec, I think). As for what Manny "sells", perhaps they're things that might make the journey safer? Client goes for a walk, they get a walking stick. Client catches a boat, he books them quick tickets. The crate full of foam might suck, but it's better than getting eaten by demon beavers walking in the Petrified Forest and takes the same amount of time. The more they qualify for, the more money they spend, the better the sale, the better Manny does. ...Possibly. That's my theory, anyway.
I've always seen it on a different way. Travel agency have 2 works: A selling you a plan for something valuable B making sure what you booked in advance happens. If you take as a fact that your deeds in life are part A of their work (if you were good, you've booked a place in paradise, so to speak), then the travel agency from Grim Fandango only have to worry about part B, and make "your booking" happen.