StableEmail Quarter Top-Up

Choose this endpoint when a user wants to extend an existing StableEmail forwarding inbox by exactly 90 days and prefers the quarterly pricing tier ($2.50, saving 17% vs the monthly rate). Use over the monthly top-up when the user wants a longer renewal period with a modest discount, and over the annual top-up when they don't need a full year.

• Username not found or inbox does not exist — returns 404
• Username format invalid (must be lowercase alphanumeric with hyphens, no consecutive hyphens) — returns 400 validation error
• Insufficient USDC payment — returns 402 payment required
• Inbox already deleted or permanently closed — returns 410 or 404
• Payment network or processor error — returns 500