Hi Dan - Sarosha here from Aiva Pop Ups :)
There are a few downsides of using an iframe/div; commonly slowing down a website and/or blocked by ad blockers.
You can find many non-iframe based popup apps on the Shopify app store. With ours, you can create dynamic, branded designs in minutes - for free. And they don't slow your site or get affected by ad blockers.
We're available to help on Live Chat if you have any questions!