Please note that most of the main email providers like Google, Yahoo, etc. will always consider terms like USPS as spam and this way, some of your confirmation emails will end up in your customer’s Spam folder.
This can be fixed by your customers by adding your email address to their address book or checking your email as not spam, using the ‘This is not Spam’ option. However, they will need to actually see your first email in order to do that.
Therefore, we always recommend that you create your email wording leaving out the carrier name and adding the tracking number instead and this should prevent the email from ending in your customer’s Spam folder.