Envato affiliate program has nothing to do with this. Doesn’t even make sense to include envato affiliate link on product description page.
What I’m talking about is including a link to a 3rd party (in this case Twilio) with referral code and informing user that signing up through that particular link would allow him/her to claim 10$ sign up bonus that could be used to test the 3rd party service and the product user purchased on codecanyon (e.g. WordPress plugin that lets you send SMS message through Twilio).
Reason why I would be interested in doing that is not for personal gain (which obviously comes with all referral/affiliate linking), but to make things easier for person who is purchasing or planing to purchase product since he/she can claim that bonus and get started without paying anything extra to test it before putting it in production.
With most paid 3rd party services user would have to purchase plugin and then go ahead and purchase the service (service plan, put money in to virtual wallet, etc), so providing a way to claim a free credit to play around makes a lot of sense (at least to me)