Allow to ignore 'Sync Next Due Date' for certain TLDs/registrars
Some TLDs (like DK Hostmaster) doesn't work like most other.
In our case, we want the domain to be active in WHMCS even though we're not billing the customer for renewal of the domain (as this is done by DK Hostmaster instead).
We don't want clients to receive invoices for 0USD (or any other currency), so the only real way would be to have WHMCS to somehow ignore the 'Sync Next Due Date' setting. Currently, we set the Expiry date to 01/01/3000 + the amount of days specified in 'Sync Next Due Date'. This sets the Next Due Date to 01/01/3000, which our Sync function looks for (for the domains that we don't want to bill the client for).