So after talking to AMCAS, I've confirmed that you can send the transcripts to their street address if you are using FedEx. Just make sure to mark them as following:
AMCAS, Attn: Transcripts
2450 N St, NW
Washington, DC 20037
For some reason the person on the phone didn't tell me the "NW" part, but it's essentially the same street mailing address as AMCAS and when I googled the address, it seemed as though the "NW" part was essential, so I will be including it.