Google Analytics Tracking Fix - Multi Domain

This is a once off job.

We require a fix for our google analytics tracking.

Here is the situation.

We have two websites. One running on Bigcommerce and another running on Shopify. We have made changes so that if a customer goes to Bigcommerce or Shopify site, and they decide to make a purchase, the checkout process happens in Shopify.

Here is the process:
1. Customer visits Bigcommerce website
2. Customers adds item to cart and clicks “proceed to checkout"
3. In the back end, a Shopify cart is created and is presented to the customer
4. The customers makes payment in shopify order page and the order appears in the Shopify admin

The customer can also decide to go to the Shopify home page and checkout within Shopify.

The problem is that our Google Analytics does not know if a purchase was made or not, Since google analytics is connected to the bigcommerce store, when a customer makes a checkout it does not appear in google analytics since the checkout was made on the shopify side.

Our google analytics is connected to google shopping ads, and since there is no sales data, our google shopping ads are not as optimised as it could be.