Make "Disable Auto Renew" clearer
"Disable Auto Renew" function is among the DNS registrar services but the WHMCS documentation states:
In my experience a registrar might sub-contract another company to make their registrar module for WHMCS. Not everybody feels the obligation to read through a complete end user documentation to understand the inner workings of WHMCS. In my case the sub-contracted company of my registrar did not do so and developed their WHMCS module wrongly.
Contrary to your documentation, they interpret that "Disable Auto Renew" is referring to the registrar function and directly switch the registrar's auto-renew on and off with it (reverse logic). This is partly their blame not taking the time to read your documentation and not taking a small client serious, but it is also WHMCS that is to blame because they put "Disable Auto Renew" in the wrong section of the form as if it belongs to other the registrar's features (DNS, E-mail forwarding, ID protection). The registrar's features indeed could be switched on an off with these, except Auto Renew which should not be touched because it messes up the invoice workflow causing domain renewal without invoice.
Please could you place this WHMCS feature in a different section to make it more intuitively clear that it is not in the same category as DNS, E-mail forwarding and ID protection! May be you could even consider not to use the term Auto Renew at all, not to confuse it with registar's. Also in your developer documentation please, explicitly refer to this feature and its true meaning from a developer perspective.
It is now for years I try to convince my registrar that they made a mistake and they simply pass it on to their sub-contracted developers and it gets never fixed, because they don't understand, likely not use WHMCS and because your form layout and labelling is just too tempting to make this error. Lots of correspondence hours have been lost of this and the added frustration to manually override the error. At least WHMCS could take it serious, please.