If the customer is having issues receiving emails with the invoices, you may need to update DNS Settings with SPF Records to confirm that Work 365 can send emails on behalf of your domain.
->Go to Admin Hub> Email Settings.
Ensure the domain you are trying to send emails on behalf of has been added as an SPF record. (There's an option to update SPF records on https://mxtoolbox.com). Further steps listed on this article to add SPF records.
Keep in mind that SPF records validate that we are legitimate email senders on your behalf, therefore, they highly recommended to improve email deliverability. There are cases that may need to add SPF records as well as DKIM records for the email to be unblocked.
Note: This will confirm that everything is setup correctly on this end, however, the email delivery is not guaranteed since it's possible that the configuration on the customer's side is preventing the delivery. If this is the case, it is recommended to see if there is a security solution implemented by the customer which could be blocking the emails from being received.
One can verify that emails were sent by going to Work 365 > Reports > Email Delivery