I've never requested one, but it's definitely something to check into. I have entered a support ticket and will update this thread with any response.
Got a response faster than I expected:
Hello,
Thank you for contacting NameJet Technical Support. I'll be happy to work with you.
Yes we have the ability to send you invoices. Please let us know the transaction you looking for and we can attach it to the ticket.
If there is anything else we can help with regarding this situation, please let us know.
We appreciate your business, and thank you for choosing NameJet as your source for aftermarket domain names.
Best Regards,
James S.
Technical Support Team