[Plugin: WooCommerce] Variation & Fixed Prices?

Hi,

I am using WooCommerce and I have a product that has add-ons.

I currently have attributes and variations setup so that the price is determined depending on which add-ons have been chosen.

However i would like to have a base price set which the product can be purchased for and then if the customer wishes to buy an add-on the price will change accordingly upon there selection.

E.g.

[Base Price] £349.99 (Customers can purchase this as a standalone product)

[Optional Add-on] £49.99 (Can add this to there cart which then becomes £399.98)

Anyone know how to do this?

Thanks in Advance

Harvinder