Retired GM Parts Manager here. FS87 is correct. GM would put a tag or sticker on them and out the door they went. Sometimes they came in bent on the edges. If you box them they are large but not heavy. UPS will probably label them as oversize. Tough call there.GM shipped them to dealers un-boxed, as in "bare", via the normal freight lines before we got the dedicated-carrier situation. Just attached the shipping labels and they were gone. Due to their size, a normal LTL freight line might be best. Check to see which ones ship to the destination zip code. UPS and FedEX have "freight" divisions now, which might also be an alternative. Might need to investigate "unattended delivery" options, too?
In another forum, it was mentioned that Fastenal will ship for low costs, via their in-house freight line. In the case of the fender skirts, securing them to a pallet might be necessary rather than boxing. Their freight line goes just between their stores, so you might need to drive a bit to find one of their stores near you and the shipping/destination zip code. Not sure of any possibility of tracking such a shipment for possible delivery time, so inquire about that, too.
Please let us know what you determine, as to the shipper.