FedEx Freight has a flat-rate freight program that ships up to 1200lbs for a specific price, based on distance.
You must first set up a FedEx freight account. Then request the boxes, then place the order for a pickup. FedEx' customer service is a nightmare and the process is time consuming to configure, but once you're in, it's a very good deal to ship this way.
If you are an 8 hour drive, FedEx considers the shipment "Zone 2" which should cost under $200. Considering that nearly every shipment I send from SC to Boardsort runs about $1.00/per pound, at 300 lbs, you'll be paying only about $0.66. If you can wait until you accumulate more, the per pound price drops. If, say you can fit 600 lbs into their box (it is a gaylord sized box) the price ends up being about $0.30/lb.
Look here for more info from Fedex.
https://www.fedex.com/en-us/shipping/fr ... boxes.html