Other than someone possibly figuring out the merchant's E-junkie Admin login and password, so in that case they would be able to log in as the merchant and create a discount for themselves or modify the item price, it is not otherwise possible to "hack" E-junkie's buttons or cart in order to place a free/discounted order.
I can confirm the link you provided above does set the proper referral cookie, so you should earn commission from any buyer who clicks that link. The fact you got a referred-sale notification indicates the referral worked correctly; however, you will be notified of all converted referrals even for orders where no commission was earned, due to the price being zero or too small to calculate a commission (e.g. $0.01 price), or due to no affiliate share % applying to that sale.