You need to have 1 USE slot left.
Then accept the Character slot quest, and it'll say : you need to have atleast 1 slot in your inventory. But you WILL get the coupon, and the quest will still be there. Use the coupon, and accept the quest again....
The End of an Era - A Final Thank You.
Unlimited Character Slot Coupons!
This will get you unlimited Character slot coupons rofl