No. Not all schools use AMCAS for their application or the letter service. The following schools do not participate in AMCAS:
Texas A&M University System Health Science Center College of Medicine
Texas Tech University Health Science Center School of Medicine
Texas Tech University Health Sciences Center, El Paso, Paul L. Foster School of Medicine
University of Texas Southwestern Medical Center at Dallas Southwestern Medical School
University of Texas Medical School at Galveston
University of Texas School of Medicine at Houston
University of Texas School of Medicine at San Antonio.