Hm, then you could simply leave the discount code off the button URL, and tell buyers they can enter the discount code manually in their cart to get free shipping on orders over $80.
As another alternative, you could simply have a completely separate E-junkie account for each site/product line. One account would have the automatic shipping discount on orders over $80, and the other one wouldn't. If they would both need to send payments into the same PayPal account, you would need to add a Secondary Email to your PayPal account and then use that for the new E-junkie account's PayPal Email setting.