Run the engine before you take the sample to get the oil at operating temperature (driving about 20 minutes should do it). Getting the oil up to operating temperature should help cook out any normal moisture or fuel build-up.
Some post offices mistake oil samples for hazardous material. Oil is not hazardous material. Blackstone Laboratories' oil sample kit meets all postal requirements. To see a letter you can give the post office to help reassure them, please click here.