Is there a system that can get information to someone, maintaining anonymity for the sender the whole way through? Like having an open drop box where you’d be able to put whatever documents you want into it.

Yes many journalistic organizations have secure drop-boxes for this purpose. You have to either trust that their drop-box doesn’t record your IP address/other info OR use an anonymity system like Tor or I2P to make sure whatever they record doesn’t reveal your identity.