When submitting through optomcas, you are given 2 options for LOR submissions:
1) Electronically: you are asked to provide your prof's/optometrist's email address to optomcas, and they will be emailed with instructions on how to submit the letter. You will be notified when the letter is recieved by optomcas.
2) Paper: I didnt submit any of my LORs this way, so I am actually not too sure how this works. I assume that the process is similar, but your prof would physically mail a hard copy of the letter to optomcas instead.
Personally, I think the electronic method is more convenient, since you dont have to worry about your mail being delayed or lost!
Just follow the instructions on the optomcas site 😀