Ash here from the Shopify team.
I'm happy to help out with this request! Some theme's come with the ability to add custom content directly to the footer. Depending on the current theme you're using, this may be a default option! May I ask what theme you're currently using? Also, could you please send a screenshot showing exactly where on your site you would want to add this contact information?
I look forward to hearing back so we can continue to work through this together.