Hi there drarnab,
If you purchased an iPhone from the Apple Store, you will need to reach out to that location to see if they are able to send you a copy of the invoice.
If you've ordered the iPhone from the Apple Online Store, please reference the information below as it should help:
My invoice has not been shipped with my order?
Within 2-3 working days of your order being shipped, you should receive an official Apple invoice by e-mail.
If you do not receive this invoice, or if you require an additional copy for your records, you will be able to print your invoice on our Self Service website.
If you need assistance with viewing or printing your invoice, please contact an Apple Online Store representative 8000 444 0396.
That information can be found here: Payment, Financing, Refunds and VAT - Shopping Help - Apple (AE)
If you purchased your iPhone from a different location or website, you'll need to reach out to that location or provider for more details o how to obtain your invoice.
We hope that information helps.
Take care.