The best way to mail tax returns to the IRS when the tax returns are due is through certified mail.
Sending the tax returns through the mail through certified mail makes sure that the IRS gets the tax returns on time and shows that you sent it.
The cost to send the tax returns to the IRS through certified mail is the best way and surest way to get the tax return to the IRS and proves you sent it on time.
You then get a record of the IRS getting your tax returns on time so that they cannot say you didn't send your tax returns.
I've sent tax returns through regular mail and it's gotten lost before but when sent through certified mail the tax returns have always made it to the IRS offices on time and never had a problem doing the tax returns that way.
Certified mail is always worth it for important mail and documents such as IRS tax returns.