Grandmother, on Medicare, was diagnosed with bone cancer. She is receiving chemo with physicians order for transportation. Why does she have to pay for return?

Her age is mid 70's. She has a previous hip replacement, hardware in hip has came apart manipulating mobility, severe constant pain, ambulance will transport to cancer treatment received at center, but she has to pay for return transportation if her cancer doctor appointment follows her treatment at the same center even if not reloaded for appointment and with letter from physicians requesting ambulance transport, she has medical necessity but still pays for transport, are there other options?