Adding PayPal PHP code

I am joing the Shareasale afilliate program as a merchant and need to add some code to my THANK YOU page so that any sales are credited to affiliates.
I don’t know where to add this code…
I added it to my page under html but it appears in the page itself.
see:
http://handcraftedcoops.com/thank-you

Should I be adding it to Appearance/Editor/Templates?
I have so many PHP templates listed, not sure where to add code.

Thanks so much for any assistance

Drew
Brooklyn NY

here’s the code I need to add:

[Code removed – Please use pastebin.com for long code excerpts]