Answering this needs the origin and destination pair, because lane behaviour varies far more than carrier marketing suggests.
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.
On the detail: asking a supplier to hold a shipment during a heatwave is a reasonable request that the better ones honour, and it costs a week rather than an order.
Solid-state stability of lyophilised peptides at ambient is well documented in formulation studies, with residual moisture as the dominant variable.
Nothing here is medical advice, and research-use compounds are not approved for human use.
Ask for a hold during a heatwave. The better suppliers agree without being pushed.