Can a distributor deny to provide the “Authorization letter for sellers”?

I purchased a few items (body deodorants) in swansonhealth.com to export and sell online overseas. Amazon has asked for the authorization letter for sellers to confirm the authenticity of the products that I have purchased. From my understanding, the letter should be between the manufacturer of the product and the distributor swansonheath.com, which confirms the authenticity of the products being sold on their website.

I have emailed swansonhealth asking them to provide me the document and they refused to supply the document. That was the response: “I do apologize, but we do not allow the resale of products purchased through us on Amazon.“

Can they refuse to provide this letter? Now I’m afraid they might not have it and are not an authorized distributor of the brand.