Don't charge me if I run out of burst capacity

Hi, I am the owner of a $20 per month Linux instance and I also have a tight budget

If this instance uses up all the burst capacity - so be it : don't tack on more compute power and charge me extra

If there any way for me to enforce the above preference?

I use LightSail so I know it will set the upper limit at baseline but: Is $20 per month an upper limit or an estimate? I'm at the beginning of the month so I want to know if $20 will be the exact charge in 30 days or ~$20. What's the range of minimum usage to peak load, is it $15 - $25? Is it possible for peak load requests to run me a, lets say, $80 tab for my instance?