Hello Holly,
It really depends what is the user experience in terms of page flow of your check-out process. Does the user navigates from your domain to your payment processing service's domain during the checkout? On what domain is the confirmation page located exactly.
The question here is not so much about which eCommerce tracking code you should execute (there's only one form of eCommerce tracking available) but whether you might need to implement cross domain tracking to ensure that the transactions you record get tied back to the correct visit. This
page has detailed information about the eCommerce tracking code. And in your case, since it seems that you use a 3rd party shopping cart, you can review this
guide to implement cross domain tracking.
We don't really have a tag builder for eCommerce transactions because the values you need to pass are dynamic, and depend on which products the user has bought. You refer to the code sample provided in our development guides and ask your wed developer to dynamically pull in the appropriate values.
-The Analytics Academy Course Staff