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?

[AlexandraR]

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