I used Interfolio to add a few supplemental letters to my app. that came in after my school's committee letter had already been submitted, and I had no problems at all with the service. I've heard horror stories on SDN of things going wrong (as there are horror stories on here for just about ANYTHING going wrong - ususally its the extreme minority), but for the most part, its pretty easy to use and quick, especially if you opt to do electronic submission of the letters.
The process of submitting via interfolio for the recommender may be a little complex, so make sure you read up on it on the site and give them detailed instructions.
The 2 science, 1 non-science requirement is pretty standard, as people have already said. I would also be sure to ask any PIs you've worked with for recs, if you've done any research. Some schools (read: Harvard) want a letter from anyone you've ever done research with. Also, people who can write about any major ECs on your application are always helpful.
A little anecdotal bit: I was a non-trad and non-science major, and I did not have the 2 science, 1 non-science requirement exactly (1 PI, 1 science prof. who was actually my non-science BA advisor, and 1 volunteer supervisor), but my Pre-med committee letter did the trick for every school I applied to. Its a moot point for you, as you're going to fly solo for your initial letters, but I just thought it might be worthwhile for others to know that the 2-1 ratio is not a hard and fast rule.