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?

[Texas Roger]

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.