How to successfully set up Affirm promotional messaging on a site?

I am trying to setup Affirm promotional messaging on a shopify site. Unfortunately their support is not very knowledgeable and has not been able to help much with my issue.

At first I tried their app https://apps.shopify.com/affirm, but it did not work and based on the reviews I was not surprised. So then I tried to manually add the JS and HTML to my theme files as indicated in their documentation here: https://apps.shopify.com/affirm

After a lot of troubleshooting I think I may have narrowed down the problem to be with the way my theme has the css selectors setup (they use # so for example class=“#price-value”). Short of getting a new theme, does anyone have any workaround suggestions? Does anyone have experience adding affirm messaging to their site?