Maypole is wrong - distance selling rules provide consumers with an unconditional right to cancel in most distance sales.
One of the exceptions to the right to cancel is where the goods cannot be returned by reason of their nature. You will be able to argue that bath bombs, for example, simply cannot be returned once used.
However, the consumer can returned them unused, and you cannot insist that they are returned unsealed. There is no exception due to hygiene, and the consumer's right to cancel is not affected by your ability to resell the goods.
You have to refund all costs, including postage. You can stipulate that the consumer must pay for return postage, but this has to be incorporated into the pre-contract information that you provide.
The OFT website (
http://www.oft.gov.uk) has some information for businesses on distance selling, and Business Link is bound to have all the relevant information, or links to it, so if you haven't already, visit
http://www.businesslink.gov.uk.