1.6.0

What's Changed

• Change 'product_image_url' to 'product_image_asset_id'
• Change General itemization 'line_items' to 'items'
• Add metadata to car_rental
• Add optional 'seat' string to flight segments
• Add optional metadata to tickets (for rewards #)
• Add optional metadata for flight segments (for e.g. meals)
• Make all fields optional on address
• Change itemized 'subtotal' field to 'amount'—so 'total' and 'subtotal' should appear only in the receipt header