Too late now, but I wouldn't have sent them any other way than signed for next day delivery - it's no different to sending cash through the post. If you still have the receipt, you just might be able to track from the point of sale, but I wouldn't be too hopeful - and unless you declared the value of the content, Royal Mail will only compensate you up to about £20 for the loss, and you'd be asked to provide them with proof of posting before you could claim from RM.