It can be done in a couple of ways, but, you have to look in to which way is the most cost efficient.
First of all, you can actually use the USPS (yes, really!). It's called "Priority Mail Open & Distribute" (PMOD). You place your trays in either yellow priority mail sacks, or priority mail boxes (both provided by wherever you get your postal equipment from). Your cost is by the weight of the sack or box, and, which zone it is going into. Depending on how heavy your mail is, and, how far you have to ship it, it is possible to get your mail to it's destination faster, while paying less over all postage (don't forget, inside those trays, which are inside the PMOD sacks or boxes, you are getting a DDU destination entry discount).
Secondly, you can have a third party private shipper (any trucking company) transport to the DDU (we usually use Fed-X Freight).
Thirdly, if the DDU's are local, you can have any courier service transport for you, or, even Uncle Billy in the back of his pickup truck or van.
You just have to figure out which way works best for you.