Invoices created before Due Date

[ptn]

Am looking to create a invoice on a set date each month but currently multiple are created throughout the month.

When adding a billable item with a due date of the 14th and invoice before set to 14 days on the 1st of the month a invoice is created which is correct and how I would like it. The issue comes when a new billable item is created between the 1st and 14th of the month a new invoice is created the next day. For example if I add a item on the 5th with a due date of the 14th a invoice is created on the 6th which is less then 14 days before and caused the client to get multiple invoices between the 1st and 14th.

Is it possible for the invoice to only be created on the 1st and anything added between the 1st and the 14th to be invoiced the following month. If a billable item is added between the 15th and last day of the month it works without a issue.

I could set to invoice with the next invoice but then if the client has a extra invoice before the 1st such as a domain renewal on the 20th it caused the billable items that month to be invoiced at the wrong date. The billable items are linked to a product but there is no via to link them to a product.

The items are added to WHMCS via API

Thanks to anyone that can help