With Shopify, most people use their online store sales channels. However there is an option where you can just generate a buy button and embed into an existing site, just like PayPal button. To set this up, follow the steps below.

Step 1. Login to your Shopify admin account and click on the Add (+) button right next to Sales Channels heading.

Step 2. Click on the Add (+) button of the Buy Button.

Step 3. Click on Create a Buy Button.

Step 4. Select the type of Buy Button that you would like to create.

Step 5. Customize as needed and click on Next button.

Step 6. Copy the code and add it to the website you want it to display.

Step 7. Next is to integrate Shopify with OSI Affiliate Software. Login to your OSI Affiliate Software and click Set Up Software.

Step 8. Click on Link Software to My Store.

Step 9. Select Shopify from the drop-down list and click on Copy button.

Step 10. Login to your Shopify admin panel and click on Settings then Checkout.

Step 11. Look for Order Processing and paste the code to the field provided for Additional scripts.

Note: If there is an existing script, do not remove it and just place the code below it.

Step 6. Click on Save button after adding the code.

Note : Shopify allows you to use these variables:{{total_price|money_without_currency}},{{subtotal_price|money_without_currency}} and {{order_number}}.
That's it! You are now integrated with Shopify.