First, be sure that they've checked their spam folder. It sounds simple, but it often solves the problem! 

If they're still unable to locate the invoice, you can forward it to them from your own email address. All administrators are BCC'd on invoices and receipts, so you can search your inbox and resend if necessary. 

If you're still having trouble, click the chat icon in the right-hand corner of the screen, and a member of Customer Success team will be ready to help.