You don't understand how the diplomacy skill works, as many people have pointed out. It's not mind control.
Think of the most charming person you've ever met. Someone you really LIKED listening to and whose opinions just made sense. You'd want to help this person in ways you could, but you wouldn't ruin yourself doing it. Weaker willed or gullible people (low wis/low int) might let themselves get totally swindled, but a high level wizard won't (unless you design him to be exceptionally gullible).
Providing aid doesn't mean doing everything the other person wants. I donate money and food to aid the less fortunate. I don't move my family out of my house to let the homeless live there, though. I'm willing to help, but not at the expense of my or my family's well being.
Personally, when I DM... I listen for the 'intent' of the interaction. I'm looking for good ideas, reasonable courses of action, explanations of events and situations, etc. I'm not too hung up on how well it is delivered; I'm not concerned with word choice or flair. While all of those things are great! They’re not what matters for how the NPCs react.
After all that, I set the DCs based on the reasonableness of the situation.
"Good sir! if I may have but a moment of your time? Would you be so kind as to pass alone this note to your master? It would be most gracious, and I'm sure he'd be pleased. /wink"
and
"Hey, take this letter to the duke; he needs to check it out"
Have the same DCs.
This is an probably the best way to handle it. I'm not in favor of the "I tell him to do this, *Roll*" method, but I'm also not in favor of requiring an inelegant player to come up with a flowery description. Go off the idea, even in plain language, and the check. If you want to provide a (small) bonus for exceptionally well crafted arguments that's fine, but unless you want to overhaul the entire system.
A little update since i talked to players and solved the issue.
After talking to them about my concerns ans showing them claxon links they all thinked that the better course of action was to use the alexandrian rule (which they liked a lot) with a little (not much) of bonus/malus given by the dm if the interpretation of the character is particulary good/bad.
So everyone is happy and che campaign is going on without problems.
Thanks again to those who understood what the problem was and helped me with advices, in particular to claxon for the links and to kolokotroni who gave a me totally different point of view on the problem, that helped a lot.
Notwithstanding that there was, in fact, no problem with the rules, I'm glad your group is satisfied and back to having fun.