($9 x 2) => $21 - $18 = $3. You can pay $2/GB per month, and they let you buy .5GB? That's crazy they don't force whole increments. Also, you're only using 3GB per month of data? That's crazy too!
To be clear, the minimum increment one can purchase is 1 GB.
We're in a bit of a weird situation where we aren't in the US the majority of the time, so most months we don't use any data off our US plan.
If we do end up in the US then that bit of code keeps buying us more data 1GB at a time as needed.
Another cool thing is that our USMobile eSIM uses the 4G data from our local eSIM so that we can still make/receive calls/SMS on our US line from anywhere. It treats the local 4G like WiFi Calling. Pretty dope.