Accepted answer
The relevant physics is that a single phase-change pack in a thin-walled box holds below ten degrees for one to two days in a twenty-five-degree ambient, and considerably less at thirty-five.
That is the argument for shipping lyophilised. A dry powder with low residual moisture is chemically stable at ambient for months; the degradation pathways that matter need water.
What a cold pack actually holds, and for how long
| Packing | Ambient | Time below 10 °C | What that means on a 12-day lane |
|---|
| One phase-change pack, thin-walled box | 25 °C | 1–2 days | At ambient for roughly ten of the twelve |
| One phase-change pack, thin-walled box | 35 °C | under 1 day | At ambient for essentially the whole lane |
| Two packs, insulated liner | 25 °C | 2–3 days | Buys a day; does not change the conclusion |
| Lyophilised solid, no pack | 25 °C | not applicable | Dry powder is chemically stable at ambient |
| Anything in solution | 25 °C | not applicable | A different risk entirely; hydrolysis proceeds |
A liquefied pack on arrival is the expected outcome rather than evidence of a problem. A single-use logger, not a heavier pack, is what turns this from speculation into a record.
The relevant detail is that a single phase-change pack in a thin-walled box holds below ten degrees for roughly one to two days at twenty-five degrees ambient and under a day at thirty-five. On a twelve-day lane the material is at ambient for most of the journey regardless of what was packed.
Solid-state stability of lyophilised peptides at ambient is well documented in formulation studies, with residual moisture as the dominant variable.
The caveat is that a warm transit is survivable for a solid and not for a solution, and the difference is not marginal.
Lyophilised solids tolerate ambient transit. That is the whole answer for most orders.