vtiger send email reminders to due invoices and past due invoices

Avg Bid (USD)
Project Budget (USD)
$30 - $250

Project Description:
I have a working self hosted (on VPS) vtiger CRM

I need you to code a method to send a particular email template, based on the status and due date of the invoices.
The email will be sent to the contact email for the contact on the invoice, the email should contain some text the invoice number and total for it.
The invoices with status Sent will receive an email 30 days before the due date marked on the invoice, then 15, days then 5 days, then 1 day.
The invoices with status Delayed will send an email template for delayed cases, every 3 days with the invoice number and total too.

Note, on invoices with Status Sent, and due date less than 30 days, only the 15 days reminder will be send, and if the due date is less than 15 days it will only send the 5 or 1 day reminder.
Note on totals, some invoices are in USD some in other currency, the email should include the invoice number, and correct total as the invoice depicts it.

You will test this so we are sure the reminders and cron jobs work properly.

This will be done on a working production CRM , so all caution must be taken not to interrupt the daily CRM use, which is minimal, but it is important.

Skills required:
CRM, PHP, Software Architecture, vTiger
About the employer:
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