Adding Variable/One-Off Items

Adding Variable/One-Off Items

Hi,

What is the best way of adding Variable/One-Off items to an (already existing) invoice? I have created an invoice, but forgot to add Shipping Costs. Now I want to add the shipping costs, but this is a variable cost of course depending on weight/country - so I don't necessarily want to create an item with a fixed cost. But I cannot create a 0.00 rated item, which I can amend at the time of creating/updating the invoice. What is the best way around this?

Thanks,
Anthony