Hi @angierich08,
Great idea to share a customer sentiment towards your business. I am not sure how the letter came in, but you can drop blocks that can be customized in marketing campaigns. The 'getting started with Square Marketing' is a good article to refer to regarding this.
A couple of suggestions: If it was a physical letter you can scan or take a photo of it then add an image block in your marketing campaign and upload it as a custom image. If it was an email, you can copy and paste it to a text block or take a screenshot and put it into an image block.
