USPS can sometimes be the least expensive option but I often find that UPS ground can do it for $5+ cheaper when the package goes above 5-8 pounds.

I use PayPal which also gives you a discount, you can quickly change providers to see who is cheaper:

Create Shipping Label - PayPal

Very important that size/weight are key, cut the box down to the smallest size possible, remove wheels if necessary to get the box smaller, making sure there is absolutely no dead space in the box... remove batteries or anything not necessary to ship that would increase the weight... sometimes I will split into 2 different boxes to ship lower weight for significantly less cost... I want to say the original quote was close to $35 to ship as a single box, but then it was $15 and $8 to ship separately... most of the cars I ship tend to average around $20 - $25 to ship but it all depends on the distance, if they are in the same state it might only cost $10 to ship, but if going to Maine, the same box might cost $40.