Paywhirl can be used across many content management systems including Shopify. The "paste anywhere on your site" verbiage is used since the widget can be embedded anywhere that you can paste HTML.
If you're on Shopify, though, we make it a little easier for you. If you haven't already, you'll need to install the Paywhirl app for Shopify.
You can install from here:
Once you've installed the app and have configured your widget in Paywhirl, you can create a "page" in Shopify. Now switch to "Show HTML"
Now you can paste the embed code that we have generated for you. That will add the widget, payment form, password reset, login, or gift code widget to your site.
Note: When you create a plan in PayWhirl, a "hidden" Shopify product is created. This is used as reference for when we place orders in Shopify on behalf of your customers. Do not delete/unhide this product or orders will not go through into Shopify from PayWhirl.