Add a product to a cart and return its checkout URL.
IMPORTANT: this does NOT charge or place an order. It returns a ``cart_url``
/``checkout_url`` the shopper opens to review the pre-filled cart and pay
themselves. Use for "add X to my cart" / "I want to buy X". For multiple
items in one cart, use create_checkout. Verify availability with
check_stock first — adding an out-of-stock item wastes the shopper's
click-through.
Args:
sku: Product SKU (from list_products / search_products).
quantity: How many (default 1).
Connector