Instead of quests or missions, there is a more general concept, a Contract.
The contract can contain sections specifying different kinds of tasks, conditions and results.
Tasks could be e.g. transporting a package, assasinating someone, building a castle, renting a plot of land, declaring war, and so on.
Conditions could be a time frame in which to do the task, or an event after which the task should be done in some time frame, etc.
The result could specify what happens if the task is or is not carried out. It could involve transfer of money, or perhaps a contract carried out by the other party.
Contracts should if possible be designed so that both parties can feel safe when using them. So if there is a relatively small monetary penalty for not carrying out a task, the game could reserve that money from the task doer until the task is completed. Or the contractor could be indebted to the contract creator (damn, what's the right term for that? Jae ;)
For transport tasks, there should be a way to wrap a transported item in a special indestructible post package, that can only be opened by the reciever, and that automatically returns to the sender if it is not delivered within some specified time period. This way a sender need not worry about the courier or highwaymen stealing the mail, but a player (and npc) courier operated mail service for items can still be created, providing interesting economical scenarios in trade and logistics.