GET /v1/gift-cards is a live Mindbody read — the denominations a
studio currently sells are Mindbody configuration, and nothing mirrors them.
Value is what the recipient can spend; Price
is what the purchaser is charged. site_id is required whenever a
key covers more than one site (the endpoint 400s rather than guess which
studio's card to sell), and it is the 32-character Datastream site id, not the
Mindbody numeric one.