Hello and Good day,
I had trouble with this at first and Seb and the team really helped to push me in the right direction.
If you look at the DIY paint experience and edit the Cart.js file in a program (Like notepad++) you will see it list something like this:
//This method is called to retrieve the e-mail template from IntuiLab web server.
ShoppingCart.prototype.InitTemplate = function(templateURL)
//a default URL to use if no parameter is passed to this function.
Now what I find the easiest is dont host your Template on your server or anywhere else.
Go to Dropbox and create an account and drop your template you want to use in there.
From that create a link and copy it to use in the code above.
Your link will end in a 0 (zero) but change it to 1 (so it looks like https://dropbox.com//blahblah/a67fkfd=1)
Then copy over the new shopping cart.js to your experience and run in test it.
One question if i missed it are you doing hosted to have intuiface send the email for you?
Let me know if that worked for you or if I could help anymore.