If my non-promotional $18 GC expires 12/31/00, but I redeem it into my account before then, but I don't actually use it, will the total amount in my account be $18 less on New Years, or does it not expire once I redeem it into my account? Thanks in advance.

The gift certificate will expire on the designated date, regardless of whether or not it has been deposited into your gift certificate account at Amazon (happenned to me once), so it'd be best if you use it now.