It looks like we're already working with you via email. But here was our reply in case anyone else happens across this thread:
When a buyer places an item in their cart, we set a cookie in their browser to track the cart contents from page to page, so that if another product is selected it will automatically be added to the same cart for ease of checkout.
This cookie expires 10 days after the buyer's last cart view, so each buyer's cart will remember any items added to it for up to 10 days.
