Turbo Tax only calculates an estimated penalty based on your filing the return and paying by 4/18/16 ... the IRS sends an adjusted penalty + interest letter based on when they actually receive the payment. This is not an error of the program, it is the way the IRS works.
Contact TurboTax support and speak directly with a TurboTax
support agent concerning this situation if you still have questions.
See this TurboTax support FAQ for a contact link and hours of
operation -https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
