“The Collected Letters of Robert E. Howard, Volume 2: Volume 2 1930-1932” on Amazon UK. Can be shipped to a locker from Amazon and currently…

Though you would need a £10 total order for free shipping. And bear in mind that the to/from Lovecraft letters are also in A Means to Freedom (2 vols.). The above just has Howard’s letters to Lovecraft, and also his drafts for these.