Knowing your costs from day one

On our most important product we had worked out the costs involved, from there we calculated our margin, and then the price. The margins were slim as we were mostly selling this to trade customers. A few months down the line the tax man contacted us to tell us that they were re-classing the product for import duties and that we would be liable for an extra 5% on the value of the imports in the future plus we would have to cover this 5% for all past imports! This came to about £30k - what a bitter pill to swallow!
Know your costs from the beginning and in a case such as this one be sure to have signed documents to ensure that your costs don't change further down the line.