I had the same problem with a gas tank. I won it on ebay.
I had the seller wash it out with soap and water, then rinse it with water and then water and rubbing alcohol. The rubbing alcohol helped the water to evaporate. Then it was bagged in a black trash bag, and wrapped in saran wrap.
The box she used held it very securely, and was very clean outside- no holes or grease spots on it. On the packing list she just put motorcycle parts-new.
The gas tank had already had a kreem kit in it, so I wasn't worried about rust.
Make sure the parts are as clean as possible, and DRY- use a hair dryer if possible.
Don't give them any hints that it's a messy oily part, and you shouldn't have any problem.