Just sharing what my ex-colleague did.
For short trips (errands that are just a few hours), she would try to pump and bring the freshly expressed breast milk out for that trip.
Otherwise, she would prepare one bottle taken from the fridge and bring a couple of bottles and keep them in a cooler bag with ice packs. The one that is taken out from the fridge would likely be at room temperature or just slightly colder when it is feeding time. She will also remove one bottle from the cooler bag during that feed so that it would be "warmed up" to room temperature in time for the next feeding.
Hope this helps!
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