Newsletter affiliate links: Substack, Beehiiv, ConvertKit compared.

Newsletters have the highest conversion rates of any affiliate channel. Subscribers opted in, trust the sender, and are in a reading mindset. They also have the strictest constraint: once an issue is sent to thousands of inboxes, you cannot edit the links inside it.

Your audience might read that issue tomorrow, or a year from now, or forward it to a friend next week. Every link inside is immutable. If one breaks, it stays broken forever.

Why newsletters are uniquely unforgiving

Compare the failure modes:

• Blog post: broken link → edit in dashboard → fixed in 30 seconds.
• YouTube video: broken link → edit description → fixed in 2 minutes.
• Instagram bio: broken link → update bio → fixed in 10 seconds.
• Newsletter issue: broken link → no edit possible → fixed never.

The only way to fix a newsletter link is to have the link itself be a redirect you control. A managed URL. Then when the destination breaks, you update the redirect, and every historical issue containing that URL starts working again.

Platform-specific constraints

Substack

Substack allows any URL. No restrictions on affiliate links. Each link is visible in the published web version of the post too (so you can at least edit the web version after the email goes out. But the sent emails remain frozen). Substack's own click tracking wraps your URLs in a tracking redirect, which is fine for most affiliate programs but worth testing before relying on commission tracking.

Beehiiv

Similar to Substack. Any URL allowed, click tracking built in. Beehiiv's click tracking appends parameters that most affiliate networks handle cleanly. Beehiiv also has a native ad network which is adjacent to affiliate but different mechanism.

ConvertKit (now Kit)

Designed for creators, handles affiliate links well. Links are not rewritten by default. Click tracking is optional per broadcast. The one thing to watch: ConvertKit's automations can send broadcasts months after you created them. Make sure your evergreen sequences use managed URLs, or a broken link today breaks every future send.

Mailchimp and classic ESPs

Mailchimp, ActiveCampaign, and similar platforms rewrite your URLs for click tracking by default. Most handle affiliate tags correctly, but some edge cases strip query parameters. Test one affiliate link end-to-end through a real send before trusting it for a campaign.

Best practices across any newsletter platform

Managed URLs are not optional

For newsletters specifically, managed URLs move from "nice to have" to "required infrastructure." You're writing content that will be delivered to past and future subscribers for months. Managed URLs are the only way to keep those issues earning after a destination changes. See managed URLs explained if you're new to the concept.

Test every link before sending

Old issue-check habit: click every affiliate link in your draft immediately before you hit send. Verify the destination still works and your affiliate tag survives redirects. Two minutes. Prevents the worst failure mode. Sending a broken campaign to 10,000 people.

Disclosure above the first affiliate link

One clear line: "This issue includes affiliate links. I earn a small commission at no extra cost to you." Put it before the first affiliate link in the issue, not buried at the end. See FTC affiliate disclosure rules.

Monitor destinations in the background

Because you can't audit past issues, you need something that watches the destination URLs your managed URLs point at. When a destination breaks, update the managed URL's target and every historical issue instantly works again. Without any email edits.