How do I combine these functions?

[diane22]

Hi, I'm really new to this. I'm trying to add this simple coupon function (Form 2, here

http://members.aol.com/paypalhelper/simpcoup.html)

to this page:

http://fetchingdogcollars.com/1.5inch-sweetheart.htm

How do I add it while still keeping the price-altering drop-downs that already exist? Like I said, I'm really new to this, so please explain in simple terms!
Thanks!