I will be sending them to all 4 (AUC, Saba, SGU & Ross) with Interolio and I bet anything that there will not be a problem. If you want to be 100% sure then have your writer write as many as you can get them to write and seal and sign the envelope.
I had my writers write on official letterhead and gave them preaddressed Xprespost envelopes and now they are just waiting to be sent to the schools with my application.
Considering the lack of ethics in today's society, I do not blame some schools for making it hard for scammers to scam false letters, but I am confident that I will have no problems.