Bonuses are taxed at the rate of 25%. For income above $37k (single filer) that is what it is taxed at anyway. For income below that it's taxed at 15%. So it's possible come tax time you might get a little more back when you file.

Example, you earn $25,000 in wages plus that $20k bonus.

The first $9200 is taxed at 10%, then from $9200-$37,000 it's taxed at 15%. Above that it's taxed at 25%. So you'd only pay the 25% tax on $8000. You'd pay 15% tax on $12,000.

This is all federal. State might be a little different as far as the brackets go.