This kind of sounds like "In the age of telegrams, is there any need for writing letters anymore?"
A sending must target a creature with which the caster is familiar. If your local wizard just hasn't ever met the guy, that puts the stop to any kind of sending to begin with. This is probably the easiest thing -- nobody around with spellcasting ability has met the sender or the messenger, so it's just kind of a dead-end.
A sending can only transmit pure information; there's no ability to send an object or a signature. A signed contract or peace accord isn't valuable for the information in it, but for the signature that indicates it's been accepted and is legally binding. A box of objects wouldn't be viable for transmission by sending at all. Your second-easiest answer is probably to have the messenger carrying something where the actual, physical item itself is what's important, not the information written on it. (Or alternatively, the information is what's important, but as Sean Connery said in Indiana Jones and the Last Crusade, "I wrote them down in my diary so I wouldn't have to remember!")
But on that note, how much information is there, and long do you really have before it becomes worthless? How many times can you cast sending every day?
A page of handwritten text is usually between 300 and 500 words and a sending can only send 25 words at a time, so even if you can cast sending three times per day, you're looking at 4 days to a week per page. That might work for a relatively short document, but anything longer than a casual letter will probably require months of effort to transmit. Can a message really wait so long to be completed instead of just rescuing the messenger or sending a new copy by foot?
Even if the sender can summarize the text to get information across faster, it still may take weeks to even get the gist of a multi-page document pushed through sendings, let alone details. The logistical limitations imposed by daily spell slots really seem to remove this as a reasonable option for anything you wouldn't be comfortable communicating by bellowing across a field.