Helega - You point out a very obvious and day to day operational point about billable time and the need for it to be more flexible. This is a very similar issue brought to my attention a while back. The advice and work around given to the entity at the time was to modify the billable time to reflect your minimum charge or after the invoice creation from the customer tab modify the Billable Time charged in the invoice and then add a separate line for travel in the invoice relating to the date in question. The comment about auditing is not really an issue as billable time is an internal mechanism at this stage for chucking time into large invoices over a period. If Billable Time were a more core component in Manager it would accommodate some of these simple requirements. A suggestion laid out to make billable time work closely to what you are thinking and would largely be able to meet what you require can be seen here:
Unfortunately - This went nowhere