I didn't really know if this is an exploit or guide, but im putting it here anyway. Basically, you use the trial before you merge your WoW account and Battle.net account, and then once the trial is up, merge your WoW account and Battle.net account, and you should be able to use it again. This worked for me, but I don't know if it will work for anyone else. My gametime card ended 2 days after I used the trial, so I don't know if that had anything to do with it.