Yes all IRS refund checks and any checks mailed from the IRS are sent first class mail through the USPS.
After the IRS mails your refund check through the USPS it can take 1 to 3 days to get the refund check in the mail.
If you haven't gotten your refund check from the IRS after 7 days then contact the IRS so they can send out another check.
But give it at least 7 days because although most IRS checks arrive in the USPS mail within 3 days sometimes mail can take longer to get to some people depending on how far they are from where the IRS checks are sent out.
So wait at least 7 days before contacting the IRS about the check if you didn't get it yet.