Your video downloads

Let's start by proving pages on the video125.fetchapp.com subdomain can be in an iframe on Shopify

Fetch download page works fine in an iframe as shown on this test page:
(both dynamic from the querystring, and hard-coded)
http://iframe.paulannett.co.uk/?show=http://video125.fetchapp.com/get/50ebbdf6

But Fetch download page doesn't work when hard-coded iframe into Shopify. Why?

...and the test page when served from Shopify's CDN doesn't load content of iFrames whether they're hard-coded or dynamic from the querystring, even when the page they're trying to load is one that works in an iFrame on a Shopify page (i.e. the only one working above):
https://cdn.shopify.com/s/files/1/0273/5127/files/test.html?show=http://video125.fetchapp.com