When you decide to buy precious metal, you are investing in a product that is subject to price fluctuations. When placing your order, you buy at a set rate, which makes the transaction final. This is similar to buying stocks or participating in lotteries. Just as it would be desirable to return a non-winning lottery ticket and get your money back, that is not possible in this case.
It is therefore not possible to cancel or return an order. The right of withdrawal, as laid down in the Distance Selling Act (an elaboration of the European Consumer Rights Directive), does not apply.