Get online support
Hi @jakejanke,
There is no chance of overdrawing your account. If you have top-up $100, you will be able to use the service until your top-up is fully consumed. Your service will be restricted if there's no prepaid balance on the account.
More info can be found here: TPG Mobile International Roaming - FAQ
Let us know should you require further assistance.
-Ahra_G