"you shall offer of your own free will a male without blemish from the cattle, from the sheep, or from the goats." (Leviticus 22:19)
Free will offerings are as according to your faith and thankfulness. This specific payment will be listed as Church Budget which assist the local Church to run.
"When you make a vow to God, do not delay to pay it; For He has no pleasure in fools. Pay what you have vowed— Better not to vow than to vow and not pay." (Ecclesiastes 5:4-5)
This is for any special project that you may want to make an offering for. See below for options