The certified letter sounds the best to me. At least that way you have all your bases covered. Otherwise, since you say you were told the games are yours and there is no papertrail linking them to you, I would personally keep them and give as gifts or sell. Finder's keepers, especially when it comes to credit card companies - they do it all the time
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"Always learn the rules so that you can break them properly." Dalai Lama
My Karma just ran over your Dogma.