Setting up attribute to influence entire order total

Hi, I'm looking to add a percentage discount as an attribute. I want to ad an attribute that will apply the % discount to the order total (or sub-total=just the discount on products$$, not shipping).
ex.-Option to pay $60 to get %25 off your whole order.

One time select on a product info page will apply to all other products my customers will put in the cart. (probably in an attributes dropdown)

None of my products are priced by attributes, They all have a fixed MSRP, with a couple attribute price add-ons for most individual products(ordering a case of 20 is +$66 to, or specially wrapped is $2) as a separate attribute dropdown.---> but that's not the point, just adding maybe useful info. I'm version 1.39h and I don't have any add ons working. Just the stock attributes.

Other ideas that don't seem possible would be-??
--Having option selected on each page and have to find out how to only allow that attribute to be charged to the customer once, instead of paying $60 for a discount on each individual product they buy.

--Can I have a coupon as an attribute, like have a $60 attribute option give/apply a coupon or group discount?

Thanks!