The postmark on the envelope you mailed the check in will be honored by the IRS as the date you paid your tax due. Mailed returns, etc. have been piling up in trailers for months. The IRS is only beginning the task of opening all of those envelopes, so you may be waiting for quite a while before they open your envelope and cash the check.
To answer your question about TT calculating late penalties and interest--no -- TurboTax will not do that. If in fact you owe and penalties or interest it will be calculated and billed by the IRS. Hang onto the receipt and proof from your certified mail that shows when the IRS received it.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
