The tablet's battery died in the middle of a sale. Did we lose it?

It depends where you were. A basket that was still being typed is gone, because nothing is stored until the sale is charged. A sale that had been charged is safe even if the device died a second later, because it is written on the device before it is sent. When the tablet comes back on, any sale waiting to go up is sent on its own. Its own settings screen shows how many are waiting, and zero means everything has reached us. The POS and the counter tab in the docs has a section on sales that could not be sent.