It's not perfect, but here's the easiest thing I've found to create an autograded "extra credit" quiz. **Note: There was a much more elegant solution, but it no longer works and this is the easiest work around that I've found.**
Create a regular graded quiz and apply the settings as desired - Quiz Settings to Maximize Security. Make sure to add a note in the directions that the quiz will grade like a normal quiz, but after the due date it will be adjusted so it becomes extra credit.
After the due date for the extra credit quiz create an extra credit assignment worth zero points and labeled it as the Extra Credit points. Make sure in the directions to add a link to the actual extra credit quiz and explain that these are the points earned from the quiz.
In the gradebook enter in the number of points people got on the extra credit quiz into the extra credit assignment. If you have a lot of students you could also download your gradebook, copy and paste the extra credit quiz points into the extra credit assignment points, and then re-upload the grades.
Go back to the extra credit quiz and change it to an ungraded survey; this takes it out of the gradebook.
IMPORTANT NOTE: When copying your course (or just this extra credit quiz) the quiz will copy over as an ungraded survey so be sure to Edit it and revert it back to a graded quiz for the next time you want to use it!
Looking for additional information about adding extra credit?
Kona Jones is Director of the Teaching & Learning Center and Faculty Academy Coordinator at Richland Community College in Decatur, IL. She is responsible for the assessment of online courses and initiatives, providing instructional design and pedagogical support to faculty, development of faculty and student technology training materials, and overseeing faculty professional development. She serves as a technology resource for the College as well as an adjunct instructor of statistics. Her particular passion is student retention in online courses and more specifically on assessing how different initiatives can improve student retention. She has a M.S. degree in Quantitative and Cognitive Psychology, Undergraduate degrees in Biology, Psychology, & History, and is ABD in Curriculum & Instruction.