We don't file quarterly taxes, but filing this year's return we're charged an underpayment penalty. Why are we being charged this and what is it? How do we not pay?
An underpayment penalty if for not withholding enough tax or not paying estimated taxes during the year.
Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and estimated tax payments, 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. Tax Topics - Topic 306 Penalty for Underpayment of Estimated Tax https://www.irs.gov/taxtopics/tc306.html
You can pay your taxes owed without the penalty and then file form 2210 requesting a waiver of the penalty.