Some of our users have been reporting access denied errors with their externally hosted links using AWS with an S3 bucket policy.
If you have a referral bucket policy configured correctly, and you're still receiving an access denied error, add the following to your FetchApp order page template (located under "Templates" >> "Page Templates"). Place this code between the <head> and </head> tags in your template.
<meta name="referer" content="always">
This should fix the issue in most cases, but if the issue persists once you've double checked the policy and have added this snippet, please send us an email for further support.