We currently don't have a mailing list uploading tool on OnlineCertifiedMail... but don't fret!
We do provide this option at our parent website; www.LetterStream.com. The LetterStream website allows you to upload hundreds of address at a time and has numerous mailing options (and it's free to signup!).
If you need to send a handful of letters, OnlineCertifiedMail is a great solution. However, if you need to send hundreds, it might be best for you to use LetterStream.