clearing of shopping-cart when no activity

Hello,

We are using WordPress with the WP e-commerce shopping plugin.
We sell products that are unique (that means there is only 1 item of each product available).
We use the standard WP ‘Keep stock in cart for’ option and is set to 1 hour.When a customer puts a product in the shopping cart, the product is labeled ‘SOLD’ for 1 hour. After 1 hour the product is available for other customers again. We also use the plugin ‘Automatic Sign Out For Inactivity’. We also set this plugin to 1 hour. This means that after 1 hour of inactivity the user is logged out, and the shopping cart is cleared.
So it works all fine when an user is logged in.
But the problem arises when a customer is not logged in, and put a product in the shopping-cart. After 1 hour the product is available to other customers again (that works fine). But the product stays in the shopping-cart. And the plugin ‘Automatic Sign Out For Inactivity’ does not work for customers that are not logged-in.
Because the product stays in the shopping-cart the customer can buy the product, even if in the meantime another customer already bought this product.
So, what do we want: we want the shopping cart to be cleared after 1 hour of inactivity (whether a customer is logge-in or not).
I hope somebody has a solution for this problem.
Many thanks in advance.
Best regards,
Michel.