Buy one unit pack with a Stripe shared payment token: 500 units for $10.00, paid once.

More units: 500 for $10.00 a pack, spent after the included units and kept until used. Only while a pass or a card plan runs; the free plan is refused with 402 pass_required. Pack units are spent after the period's included units, Lab units included, and never expire while the account exists; GET /api/agents/usage shows what is left under unit_packs. One token pays one pack, once (409 payment_token_used otherwise). Not refunded except where the law requires.