When you filed did it say you had a tax due on the 1040 line 37? If you had a tax due how did you pay it? Your only proof of payment is your bank or credit card statement showing it coming out. Turbo Tax does not pay it. They only send your bank info to the IRS or state for them to pull it out of your account.
Are they billing you for the whole tax due or just some extra interest or penalties? It's common and normal and expected for the IRS or state to bill you for more penalty. If you do not pay enough tax, you may have to pay a penalty for underpayment of estimated tax. Even if you are getting a refund you can still owe a penalty for not paying in evenly during the year. Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller.
