Don't panic!
The fees are not charged until the listings go live so any discounts/freebies available at that time, which apply to your listing, will be taken into account then.
One idea is to schedule the items for later than Sunday, then go in on Sunday, edit the listings and take scheduling off. Then you will be charged the going rate to list at that time, ie. £0.00p
.
Good luck - hope you can get this right. I've wasted more money than I got back by doing this over the summer, so don't forget to take scheduling off before it goes live.