Proper pronoun in a response letter for a single author manuscript I am currently in the process of responding to reviewer comments for a journal article submission, and I realized I was using the scientific/royal "we" in my response letter, even though I am the only author. For example, I have written, "We thank the Editor and reviewers for the constructive comments..."

This is my first time as a single author and I know in the actual manuscript it is still normal to use "we" to indicate the author and the reader. There are multiple questions here that address that question about pronoun usage (most refer back to this question which has a great answer: Choice of personal pronoun in single-author papers), but I have been unable to find a question or an answer that directly addresses my question about the use of "we" in a response letter to the journal from a single author. I understand that since it is a letter and not a manuscript, it doesn't have to be quite so formal, but much of the content is still scientific and so I am still apprehensive to use "I" (when using a passive voice would be awkward).

So, should I continue using "we" or would it be more appropriate to use "I"  in a response letter to the journal? Additionally, would these guidelines be any different for an initial cover letter that accompanies the first submission?
You're really overthinking this. Response to reviewers are relatively informal, and frequently quite terse1; you can certainly use "I" and no one will raise an eyebrow.

1For example, if I've made a change suggested by a reviewer, I simply reply: "Done".