def fetch_billing(patient_id, data):
    if patient_id not in data:
        return "Invalid Patient ID. Please check the ID."

    patient_data = data[patient_id]
    test_list = ', '.join(patient_data["tests"])
    total_cost = patient_data["total_cost"]
    return f"Patient: {patient_data['name']}\nTests: {test_list}\nTotal Bill: {total_cost}"