Welcome to XF2 Addons. You can buy my Premium membership for $35 per year, and this gives you access to download any or ALL of the 300 add-ons that I have developed for Xenforo 2.x. In addition, Premium members will get technical support. Once the Premium membership has expired you can continue to use the installed addons on your forum as long as you like.
Wonder if it could be solved by adding a <span> tag to the add-on that shows "Paid Link" when you're hovering over the affiliate link? Seems to satisfy the requirement (this is what Sovrn/Viglink does), and would be fairly simple.