Return a payment link for the person to open in a browser: either a subscription plan, or a single image bought outright (only where price.card_available is true) — or that image's commercial license, with licence=commercial, where price.commercial is not null. The commercial license is sold only this way; Stripe's page asks the person to name the licensee. THE AGENT NEVER HANDLES CARD DETAILS and must not ask for any — hand the url over and let the person pay on the payment page. Nothing is granted until they finish. After a one-off, call fulfil_image with the same listing_id: once the payment has landed it delivers the image at no credit cost, and until then it answers payment_pending. After a subscription, get_account shows it. Every result lists the plans with their prices. Use fulfil_image instead when the buyer already has credits or already owns the image, and start_trial when they have none and have not tried the store. Refused when the buyer already owns the image or already has a subscription, so that nobody pays twice — except an image the free trial bought, which is the screen file only: a one-off for it buys the print file at the full price. Each new link cancels the previous one — any earlier subscription link, or an earlier link for the same image — so give the person only the latest.