You are definitely supposed to sign and date an official form stating that you are waiving your right to see the letter. My postbacc program told us to give these to all our recommenders at the time we asked for the letters, and they were supposed to send the waiver back with the letter.
These waivers are VERY IMPORTANT. It's not really fair, but med schools assume that if you have the right to see the letter [which you DO have, by law, unless you waive it], the recommender won't be honest in their assessment. So waiving your rights is the norm--it would be unusual not to.
If you have sent in letters already to Interfolio or another service without waivers, you MUST send them in before your med schools get the letters. I'm sure Interfolio has a waiver form for you to sign, or maybe you can ask your premed office. I don't think an email would be legally binding, so don't rely on that.