The creation of a QUIZ should work nicely to manage 'student-awarded' extra credit.
Consider setting the quiz up to only show one question at a time and lock questions after answering to improve the integrity of the extra credit.
This quiz could be setup with only one question or you could set it up using 3 questions:
You might first present an honesty statement and ask if they found the extra credit points statement - answer choices can include "Yes, I found the extra credit points." (with yes being the correct answer).
The second question might be 'where' they found it - using multiple choice.
Third question might ask what points were found (if there are multiple points to find) - using short answer or multiple choice.
All three of those questions will auto-graded by Canvas, unless set up as short answers. Either way, the gradebook would keep track for you and you could do the final awarding of points at your discretion in the future or at a certain time after the quiz closes (using available dates).
Also, setting the quiz question(s) up to reflect the number of extra credits points earned will save time in the future - maybe so many points per question -or- only points for the third question and 0 points for the first 2 questions - or any number of possibilities.
I'd love to know what you decide to do.