Conclusions

No hourly billing
Invoices are always before usage
Invoices can be paid from the balance
Invoices can be paid from multiple payment types including Bitcoin, Credit Card, and balance
The balance can be filled by Charges
A charge can have type Invoice, Balance, or Donation
Multiple charges can be paid to an invoice
One charge cannot be paid to multiple invoices
If an invoice is generated while an existing invoice is open, it will close the old invoices and add them to the new invoice