They should be checked and fabricated by trained individuals. This is one of the last arts of medicine. If the diagnosis is vague or incorrect, the device will be less likely to help. Orthoses support the whole foot, from heel to toe, not just the arch. Often with the first appointment, some kind of temporary orthotic to evaluate whether a permanent one will help is provided.
Some offices collect a deposit at the time of casting.
Though there are no guarantees with any medical treatment, most practitioners will adjust the device without a charge. Treatment not related to the device, i.e. a new injury, or continued problems needing additional evaluation, will be charged. The lab I use optically scans the casts and stores them for 10 years. They give the device a lifetime guarantee against breakage. Sometimes they stall payment until they get a statement of medical necessity or a copy of the chart notes.