On three things together: sample format and count, journey length including a realistic delay allowance, and who is physically going to handle it.
Format matters as much as count. Straws, high security straws and vials occupy a canister differently, so a capacity figure quoted against a reference format tells you little about your own. Ask for the calculation against the format you actually run.
Journey length is not the courier’s transit time. It is that plus the customs hold you have not planned for, because published hold times are measured upright and unopened at moderate ambient and real shipments meet none of those conditions reliably.
The largest unit the budget allows is usually wrong. A bigger shipper is heavier for staff to lift and more expensive to ship. The CryoStork range runs from 2 litres to 10 litres for exactly that reason.
