Cooler bags all look identical from the outside, which is exactly the problem. Four things decide whether one works.

Ask for hours, not millimetres

A foam thickness is not a performance figure. Ask how many hours the bag stays below a stated temperature, with a stated load, at a stated ambient.

A supplier who will not put that in writing is telling you they have not measured it.

Look for a flap over the zip

A zip is a straight-line gap across the top of the bag, and heat does not care how thick the panel is if the lid leaks.

An insulated flap over the zip is cheap, invisible in a photo, and one of the first things cut.

Look inside at the seams and the corners

Taped or bound seams stop the inside bridging to the outside. Radiused, heat-sealed corners are what let the bag be cleaned.

Square stitched corners trap liquid, cannot be cleaned, and are why old cooler bags smell.

Get the word “leakproof” defined

Leakproof should mean a welded, seamless liner with no stitch holes below the fill line — a bag that holds melted ice in a car.

Everything else is water resistant, which is fine for condensation and a spill and is not the same claim.

What to send when you ask for a quote

Artwork, quantity, and how long the bag needs to hold cold in real use. That last answer picks the liner, the closure and the price band between them.