I used Interfolio. I got a three year subscription for ~$35, and have my letters sent there. I am applying next cycle, but have been completing pre-reqs, shadowing, ect for the last few years. After I complete a class, or do some shadowing, I have the letter writer write the letter while I/my accomplishments are still fresh in their mind. When it is time to send my application, I just select the letters I want to sent, and Interfolio takes care of the rest.