Can I use MongoDB as the database for providing a paid service?
MongoDB is licensed under the AGPL, but the drivers I'm using are MIT licensed. Do I have to buy a commercial license for MongoDB or can I use it as a backend for my app?
The use of MongoDB as a backend database may be used for commercial web based services and does not require one to GPL or AGPL the web based service. Do note that nothing in the GPL or the AGPL prevents anyone from using the library/database/whatever commercially - just that you need to distribute the source code of the work in its entirety to people you have distributed the work to.
MongoDB recognizes that applications using their database are a separate work:
we promise that your client application which uses the database is a separate work
This means that you don't need to be concerned with the licensing of MongoDB to use it. They'll even send signed letters asserting the promise to legal departments if there are questions (and they'll do commercial licenses if the signed letter isn't enough for the legal department or you live somewhere where such a promise isn't binding).
That said, when a web programmer sees the AGPL, it is indeed right to go "wait, what?" and look closely at what is being used where and what it implies about your source code licensing.
The specifics of why MongoDB is using the AGPL rather than some other, more permissive license stems to commercial companies modifications of MySQL. For Example, Google Cloud uses MySQL in its backend. However, there have been some changes to it (disabling some features... and possibly some optimizations). Since MySQL is under the GPL and has the web services loophole available to it, it doesn't need to submit those changes back to the MySQL community.
MongoDB, by selecting the AGPL, forces that if a company was to do what Google has done with MySQL, any changes would be submitted back to the community.
This is only an issue if you have modified MongoDB from its distribution. If there are no changes to MongoDB, you may use it anyhow you like.
See also: http://www.mongodb.org/about/licensing/
You can use AGPL programs for anything, including commercial programs. If you make changes to the programs, however, you have to release these changes. As the driver can be considered "part of your work" under GPL terminology, you might have to release the source code when using an AGPL driver.
This is the reason why the DRIVER is not AGPL. So you don´t have to worry, at least as far as my understanding goes.
But again: I am not a lawyer, I might be wrong.