To be honest I haven’t had much experience with Membership plugins. However, I have been using the WooCommerce plugin as my store to sell themes…..so when someone purchases a theme, they only have to purchase it once, and they can login and download the theme. In your case you could do the same thing with software, depending on what all it is. There are also some paid extensions that you could add that will enhance the features as well. You can check it all out at – http://www.woothemes.com/woocommerce/
I just did a quick Google search and came across this – http://chrislema.com/comparing-wordpress-membership-plugins/ ….you could search for more if you need to, but the WooCommerce plugin is the only thing I’ve had experience with.