HELP please with promotion

Hi everyone. I have read thru a lot of posts to see if this has been asked but I don't see it anywhere.

I am trying to set up a promotion (similar to a Groupon deal)…the offer is : customer pays $19 for $40 worth of products.

I know how to set up the actual promotion with the code the deal company will be using…however, I don't want it to be used to pay for taxes and shipping.

For example: Someone choses a pair of $32 shoes, They type in the code…gives them a zero balance to pay which is fine. However, the problem I am encountering is the extra $8 is being used towards taxes and shipping which I don't want…that is not part of the deal…

How do I fix this?

Also, since I am asking…I am also offering free shipping if they spend $50…can this all be set up as one promotion?

Thanks in advance for any assistance. I am a newbie at this…