Potentially, but the claim needs to be defined carefully.
A mailer can provide short-term moisture resistance without being designed for indefinite water exposure. Compostable does not mean that the bag must immediately dissolve when it gets wet.
The engineering challenge is to balance:
- moisture barrier;
- seal integrity;
- toughness;
- printability;
- storage stability;
- the intended composting conditions.
The product should be tested for realistic rain exposure and delivery conditions.
I would avoid absolute claims such as "waterproof" unless the product has been tested to a clearly defined standard. "Moisture-resistant" or "designed to protect during normal delivery conditions" may be more accurate.