Sorry to heart about your issue, you can find the transaction number in your account history section on the main site where all your most recent transactions will be listed. SR will not be able to help you though with resolution as you have already finalized and released the coins to the vendor. In future do not FE for any orders and you will have the protection of escrow should you need it. As things stand you are at the mercy of the vendor now as to whether or not they are willing to work out a resolution with you. I have read a few posts recently of UK domestic mail going missing as it goes, not a good trend if it is on the rise.