Cancel your sub now, you'll still be able to play the remaining time left. When your sub ends, enter the time card code on the mogstation. Time cards are usually a code you enter so if you buy it now you can use it in three days time. I would caution you on trusting someone just because they are on the same Facebook group; it's the internet after all, anonymous people have no reason to be nice. If you feel you can trust him then that's up to you.