We don't send in quarterly estimates, but we do always pay our full taxes due on time. This is the first years we've gotten an underpayment penalty. Why?

Generally, you avoid an underpayment penalty if the total tax owed is than $1,000 after subtracting your withholdings and credits, or if you paid in at least 90% of the tax for the current year, or 100% of the tax owed for the previous year, whichever is smaller.  

Please see the attached for more information:

https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes