Yeah - it's a bit of a pain in the ass. Costs $20 or something to use ... for just one year. Plus, you have to pay $6 for every LoR you send out to every college - even if Interfolio just emails the letter (you can choose whether you want them to email the letter or snail mail the letter to the schools you're applying to)!!! But I guess it makes things easier on the profs, who don't have to write more than one letter. Still, rips off the applicant big-time.
Also, make sure you email grad schools' admissions offices to make sure that they accept letters sent from Interfolio. I think some of them don't.