This one is tough and depends on your email provider. The first thing to try is to add to your personal contact list. E-mail providers have gotten very good at blocking or marking as Junk suspicious emails with attachments. Even though you may receive our regular emails, you may have to take steps through your email account for invoices to get to your regular inbox. If adding to your contacts still doesn’t work, here are some links below that show how to mark items as safe through different email providers.
Outlook / Hotmail: