I’m sorry you didn’t get what you expected. I too will feel disappointed if the original amount advertised was not followed. However, I can forgive the brand since they issued an apology and the rest of the products were as described.
5mL sunscreen can last 4 days more or less depending on use and how big the face is. 1/4 or 1/5 tsp or 1 mL a day plus reapplications. I believe that’s enough for “testing” a sunscreen. If it makes you feel better, disclose that you received a 5mL sample.
Do people really use 1/4 tsp on reapplication? I don’t. Sunscreen is still there, it doesn’t magically disappear after two hours. Some have been rubbed of, some filters have degraded, but there is still some protection left. I usually use a small amount if it’s reapplication and I’m just indoors. I will use more and reapply more frequently if I’m continually under direct heat of the sun or at the beach.