Create a new item in your store called 'Bulky Item Shipping Surcharge'. Give it a price of whatever you want your surcharge to be, and also mark it as hidden by ticking 'Hide on Web Site' in the 'General' tab.
Then, for any product that requires the surcharge, create a new component called 'Surcharge'. Within this component, ensure it is non-optional, then in the 'Associated Product' field, select your new hidden product. Ensure you tick 'Use Associated Product Prices' and 'Use Associated Product Name'.
This surcharge will then be added to the shopping cart at the same time the bulk item is added to the shopping cart.
Then, for any product that requires the surcharge, create a new component called 'Surcharge'. Within this component, ensure it is non-optional, then in the 'Associated Product' field, select your new hidden product. Ensure you tick 'Use Associated Product Prices' and 'Use Associated Product Name'.
This surcharge will then be added to the shopping cart at the same time the bulk item is added to the shopping cart.