Hi there Oleg,
This is Evan, a Guru here with Shopify.
The checkout page is very limited in what it can have changed. So the success of the script will depend on what change you’re trying to make. I took a look at Monetate and it looks like they have a Shopify Integration listed in their help documents. Is the script you’re adding part of this process? If you could give a little bit of context as to what the purpose of the script you’re adding will be, that would be great! For more information on additional scripts you can click here. That will give you a brief rundown of how that section works, and how it affects the checkout.
As I mentioned, the checkout is limited in what kind of customizations can be made. So knowing the type of changes that are being made will help us determine how and why the script isn’t working. If you have have any other questions, or comments please let me know! I’ll be more than happy to help out!
Evan | Shopify Guru