With something like MobaXterm, they allow you to download a limited version of the product for free, and a full version of the program hidden behind an online a

With something like MobaXterm, they allow you to download a limited version of the product for free, and a full version of the program hidden behind an online account. When you pay for the software, it generates an account you can use to log into their website and allows access to the paid version of the product. Not sure if it phones home, but having a login/password combo restricting access to the paid version of your product would likely be "good enough" without having to use "check if I'm registered" online logic. Of course, if they decide to share the program then you don't have any recourse but that might not be a huge problem, depending on how honest your customers are.