Wheaton Arms can engrave your 80% lower. You cannot do any work to the 80% receiver prior to sending it to Wheaton Arms for engraving. If the receiver has had any work; machining, drilling, tapping, milling, etc, this changes the classification of the 80% receiver to a firearm. Below is more clarification from the ATF conerning 80% receivers.
80% receivers or 80% frames are not considered firearms by the ATF. You, the owner of the machined or cast part can ship via USPS, FEDEX Ground or UPS Ground.
Feel free to contact Wheaton Arms with any questions.