Write a Wordpress Invoice Plugin

I need a customize wordpress plugin. Currently, we are using gravity form. User will register on the User Registration form, verification needed, Redirected to Payment Gateway for collection (We have the existing script and it is working, we need to use PHP to create a transaction signature only, example will be provided) System will need check if this is existing customer or new customer, and to auto create an invoice basing on running serial number and email to customer, upon successful collection. Auto reminder email to customer for non-collection of payment. Store all data in database. Invoice system is able to perform manual entries, deletion and modification. Simple reporting is required. Abilities to import and export data in CSV is required. Note on our terms of purchasing: This is a simple system to replace our existing system which is half automated. 30% upfront payment, and upon testing on your website, we accept the software, balance payment and transfer of software to us. Bid this project if you are agreeable to this.,