Tier pricing issues

When I am trying to set up tier pricing if I put quantity at 2 and discount at $5 it doesn’t show up in the cart at all but then if I make the quantity in the cart 3 it takes $15 off the order and when the quantity is 4 it takes $40 off. I just want people to be able to order 2 for $15 instead of $20 but it is not working. Also what settings do I need to use to get anything more than one item in the cart to have a discount?

https://wordpress.org/plugins/sunshine-photo-cart/