Many schools have career centers that won't serve as a letter service. This is more common at smaller state schools. My school didn't have a letter service. My solution was to get individual envelopes for the schools I wanted to make sure everything was perfect for(emory and mcg) and then I used interfolio for every other school. Every school I applied to told me this was ok, and I got interviews at several of these.
One school that won't accept any letters from interfolio is wake forest. But it's not like I was dying to go there anyway...yeah I may have applied if they accepted them, but it wasn't so important that I was going to have my lor writers send off individual letters to another school. I'd go with interfolio...they are reasonably priced and you can always check the status of each letter online.