We take two measurements:
(a) When the order is generated we calculate total prints (geometries in order * the times each geometry is set to be printed)
(b) When the print is executed we take the same measurement (geometries in order * the times each geometry is actually printed out)
The number used to invoice you is the HIGHER of either (a) or (b). We charge by tiers, so these numbers go against your account and we keep track of whether you're exceeding your limits.
If something is uploaded at one time and printed another, and the latter is higher, we associate the difference to your account. We currently do not count reprints of failed items towards total, though this may change.