Add a product (by SKU) to the cart

Carts of signed-out visitors are keyed by the pp_cart cookie (set on first write) and merged into the customer cart at login. A signed-in customer who already bought the trial (or holds an unpaid trial order) gets 409 trial_already_used for traffic-trial. A dedicated port from the partner network is sold for its own term only (no term_override; each term is its own SKU), a few units per line, and only while it is on sale.