Buy and install before you fly. Activate after you land. These are two different steps and confusing them is the most common mistake.
Installing writes the profile onto your phone and needs an internet connection — which you have at home and may not have at the airport. Activation happens automatically the first time the eSIM connects to a network at your destination, and that is when the validity period starts counting.
So installing early costs you nothing: a 7-day plan installed three days before departure still gives you 7 full days from the moment you land. Waiting until you arrive means standing in an arrivals hall trying to find Wi-Fi to install a product you already paid for.