Hey @Privvos,
What kind of links are you trying to add to your estimates/quotes? We wrote a short piece about how to add social media links to sales forms. The same method should work for what you are trying to accomplish.
Essentially, while you can't add buttons or clickable images to sales forms (estimates included), you can add clickable hyperlinks your email text. If you want to segment who gets which links, you can create custom estimate templates and change the default email messaging for each version.
The article also offers some "best practices" on where, when, and how often to send these kinds of links. It all depends on your intention. For instance, if you want to promote a blog or something purely informational to customers, you can probably permanently leave these links on all of your sales forms. However, if you want to do something for marketing, you may want to throttle back. Regarding the latter, consider anti-spam policies in addition to the kind of impression "spam" will leave on customers.
I hope this helps!
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