Postage rules don't work for a lot of items. I used to use it for a particular product I sold that I used parcel post for but I could only get it to work for 2 items, not 3 or more. If you can send a lot of items for the same postage as one item you can just charge $0 for every subsequent item.
If you use GTC for your listings you could use volume discounts or other promotional discounts. Have a read of this thread on the subject:
https://community.ebay.com.au/t5/Selling/Client-discounts/m-p/2162929#M171569