I believe that the Advanced Installer guys, as you know, offer discounts for such cases, try contacting them by email.
As for NSIS, it depends on how much effort you are willing to put into maintenance. There is also WiX from the same category. Also a steep learning curve.
Installer sets, as a rule, you edit once a month or every new version of your product. I find that between these periods I tend to forget things, so the intuitive and easy to use editor certainly helps, and the free tools are not really known.
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