It was long known among the Google Ads community of Gold Product Experts that Google uses bots to add products to the cart and proceed to the checkout. However at the time it was not confirmed by Google.
While yes, this is extremely annoying and in some situations can cost the merchant lost revenues to 3rd party remarketing tools or systems where the bot can actually make a purchase.
Bad actor merchants always try to outsmart Google by not following policies. For example submit free shipping in Google Merchant Center, but then charge a fee in the checkout. Or list prices ex vat and charge vat in the checkout.
To combat this behavior, Google has built a bot to buy products and validate the price accuracy both on the product landing page and the checkout. You can recognize it’s Google by checking the IP address and they will have a user name John Smith.
What Can I Do?
First, let’s be very clear and explain what you can not do. Do not try to block users, be it bots or humans from the checkout. Google has a policy in place that states you need to allow everyone to make a purchase regardless of who they are, where they are located, even if they are outside the target country you are advertising in. This is briefly stated here: https://support.google.com/merchants/answer/4752265?hl=en
Make sure your landing pages always match your products. Show a product on your landing page that is essentially identical to the product in your product data, regardless of the user’s device, browser, location, cookies, your ad targeting choices, or any other consideration.
What you can do, is contact your analytics or 3rd party tools you are using to setup a filter to not track these users.