WHMCS offers only one invoice reminder time, which, to me, is a bit insufficient. I find that a 2 week reminder is "ok" for an annual customer, but a 2 week invoice reminder for a monthly customers is a bit overkill. And a 1 week reminder for an annual customer is not enough. I hate to charge people $500/year for service and their reminder is so short.
I think it would be great if we could customize invoice reminders based on billing cycle. For example, people who pay annually may get an invoice reminder of 30 days in advance. People who pay monthly, may get an invoice reminder of 1 week in advance.
The ability to create multiple invoice reminders (automatically) would be great as well, especially for quarterly/annual/bi-annual/tri-annual customers. I would love to be able to set at least 2 or 3 reminders for annual customers so they will have plenty of notice ahead of time of their upcoming bill.