(edit) looks like that's not the case, I'm sure I used to have to buy a second instance a few years ago if I did want to use more bandwidth that was allocated
For those curious, the overage rate is $10/TB ($0.01/GB) after the transfer included in the plan.
The smallest amount of included transfer is 1TB for the $5/mo VPS.
If you use up your monthly network transfer pool, you can continue to use your Linodes normally. That being said, you will be charged $0.01 for each additional GB at the end of your billing cycle.