First thing you do is email them a contract with a _reasonable_ consultant hourly rate. You literally handed them documentation in advance and they didn't bother to read or test it. Don't feel bad, don't get angry, get paid.
If you are contracting in a situation like this a reasonable rate could mean $250-$1000 an hour. Really depends on what kind of money your currently make.
A better approach in the case would be to offer to investigate, and provide a flat rate. You can charge $3000 flat fee, and you may solve it in 1.5 hrs.
They accepted :(.