Ok, you already alluded to the fact that others would disagree with you.
I hope your company does not have too much success. If they do, it is possible your SQL Server will not be able to handle the load, and you will have only one option...scale up; buy a larger SQL Server.
For this reason, this kind of work is most commonly performed outside the database.
Your concern regarding coupon tampering is extremely valid, and the transaction capability is also included. That being said, there is literally nothing keeping you from generating this as a Web Service, or WCF application, allowing the protection you require, transactional capabilities, and scalability. It would be completely inside the firewall.
If cost is an issue, you could initially run the service on your SQL Server; just not in the SQL Server process. Then, when your company has an explosion or grows beyond your capacity, you host the PDF generation in a web farm inside your firewall, calling it from your web servers just as you do the database today.
Frankly, it is very cool what you can do with the CLR inside SQL. This is a great demonstration of how flexible it is. But, just like using Indexed views to solve a bad schema, this is also not a great choice.
Write more...you communicate very well.