Discount Script for Shopify.

I need a script that will generate a discount when 17 numbers are entered into the Discount section at checkout. If 17 numbers are entered into the discount section and if the order value is greater than $20.00, then discount total order by 20% and set shipping price to $0.00. I currently have my shipping settings so any order of $20.00 or more gets free shipping, so the script might only need to discount the order by 20%. The script would need to work if the person enters dashes as well, as I'm asking them for an Amazon order number…which is like this… 114-1420203-7579454 Only reply if you've written scripts for Shopify before and reply with I do Shopify Scripts in your first line so I know you've read this. All others will be deleted.