The fixed amount_off coupon only nets $1 for a quantity of one, and the
offer is meant for genuinely new signups. A lapsed account that kept
several workspaces and re-subscribes through onboarding would otherwise
be charged N*price - amount_off (not $1), and a returning customer could
whittle down to one workspace to claim the discount again.
Apply the coupon only when the paid first month is enabled, the account
bills a single workspace, and it has never subscribed before; every other
checkout falls back to allowing promotion codes at the full price.
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