How can we improve WHMCS?

Currently "Add funds" invoices are generated without TAX, we request to add the TAX in that invoices,

and when the client spends those funds, instead of generating an invoice issue a receipt without TAX.

The receipts must be numbered differently as the invoices.

In Spain, there cannot be invoices without TAX and the invoice numbers must be consecutive. It's a legal requirement.

Thanks.