So this might be helpful to others in the future. Since I got conflicting responses, I decided to call eBay to make sure. Here's what they said:
Basically, since there is a bid, the seller is not allowed to offer discounts or refunds, per their policy. Obviously, you can and you probably wouldn't get caught, but the lady I spoke with said that's against the rules. Buuuuut, she said that the buyer could buy it now and then I could send an invoice and take off the shipping fee in the invoice.
Obviously this would be for very specific instances where the discount is similar to shipping cost, but it's an option.