I am old-fashioned and I will send the document both via webform and regular mail, just to be on the safer side.
Mail:
You can send the additional ID photocopy via Courier along with a short cover letter explaining the ids that you've previously submitted and the photo id that you have chosen to submit now, include your UCI/Application number/Dob in your cover letter so they can easily track your application, and you could also mention in the letter that you have submitted the ID via Webform. Granted it will cost a little bit if you decide to send it via Courier however you can breathe easy know that it will reach them sooner.
Webform:
Once, you have sent the courier, It shouldn't take much time to send the document via this step. Just an added extra backup plan.
Perhaps, any other applicants who were in the same boat as yours can share their experiences with you. In the end, you have a choice, man! This is just the route I would take
Good luck!
Kal