Considering the value of the watch, i really think you only have one option here. Call them and say that you have no proof of sending the watch for repair as it was done through an internet club, and the repairer is a hobbyist, not a business. Now await their next move. They will undoubtedly affix a price and try to get money from you. If the amount is small, pay it. If it is substantial, gasp and tell them it is a cheap replica that you received as a gift. (explaining both the declared value, and lack of shipping proof on your end) At this point the may confiscate it or they may not.
Above all, do not extend your lie any further than it has gone already.