Follow this link and review the instructions to add two simple lines of code to your checkout.liquid file.
If you are using the new checkout, please see instructions below:
On your Shopify Admin Panel, click on Online Store .
Click on Customize next.
On the next page, click the drop down menu button and select Checkout.
The next step is to add the app, click the button Add app
You will see your list of available apps, select Superbump-checkout-upsells
Don't forget to click the Save button.
That's all there is to it! You're done installing the Pre-purchase (checkout) widget!
Please let us know if you need further assistance. We would be happy to do this for you.