Answering this needs the origin and destination pair, because lane behaviour varies far more than carrier marketing suggests.
Tracking quality is the more informative signal: continuous scanning at every hop indicates a real logistics chain, and a tracking number that goes dark for a week indicates something else.
Reported ranges from Chinese origins to western Europe cluster around nine to fourteen days with tracking, with inland origins adding a domestic leg and coastal ones sometimes beating the range.
Arrhenius kinetics predict degradation rates rising with temperature, and the rates for a dry lyophilised peptide at ambient are low enough to make short-term excursions immaterial.
Customs and legal considerations are separate from thermal ones and are the more consequential risk in most jurisdictions.
Ask for a hold during a heatwave. The better suppliers agree without being pushed.
edited 1 Dec 2024 by sinead_gaffney — fixed an arithmetic slip in the third paragraph