Command or Code to Clear Cart

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Is there a built in command that I could use on my ordering template that will automatically clear the cart?

 

I have my ordering page setup so the product drop down box is not disabled when a customer selects a product using the Single Page order form. The problem I have is that this allows customers to select a product and click "Update Cart," then select a product again and click "Update Cart" for a second time when I only want them to have the ability to add a single product at a time. Otherwise the process is confusing because the customer will believe they are only selecting a single product, but the pricing shows greater than what they have selected on the page because there are multiple items in their cart.

 

The best way to solve this that I can think of is to clear the cart when someone selects a product in the drop down box of products before displaying the product that they just selected. Anyone have any ideas?

 

Thanks!