Substack Newsletter Sign Up Block Causing Issues In Mobile View

In the footer of my website, I have inserted code to embed a sign up form for my substack newsletter. 
In desktop view, the newsletter block shows up fine but in mobile view, it is too big for the page & is causing the rest of the page content to shift to the left.

Is there a way to fix this or should I get rid of the code?

Link to my website: https://www.saltbuttersmoke.com/

The iframe code has a width of 480px, so on screens smaller than 480px wide it breaks the design, you'll want to experiment with adding: max-width="100%" to the <iframe>

All sorted!

Thanks for your help!