How do you ship seats and heads?
I agree that 1 box is cheaper and through Fedex.
Not always, it is cheaper as long as the box stays under their size limit, (130") but that is hard to do with seats and other large items. You can ship two boxes under 130" for less than half that of one box that is over 130"
I ship a lot of large items, Bikes and car parts and you need to do your homework on fedex.com and enter different sizes and weights to know for sure which is cheaper, it's not set in stone, just a 1/2" difference in box size can make the difference of it costing $30 to ship or costing $100 to ship the same item depending on the box or boxes you use. The same goes for UPS and USPS which is the most expensive if it can't go priority flat rate.
130" is maximum shipping size for FedEx and UPS and as far as weight I believe 75 pounds is maximum weight. Also anything over 54" long goes over sized even 1/4" over.
To give you and idea,
A box 54 x 8 x 30 is 130" I use this to ship bikes and I have shipped seats in them but they had to be taken apart and go in two separate boxes.
A box 25 x 25 x 30 is also 130" you can "sometimes" fit two lowback bucket seats in one this size but they need to be completely taken apart and it will still be tight, but they will be over weight and cost you more for the weight.
I occasionally ship bike parts and frames to a friend in Australia which has a 79" size limit and an 1980 Mongoose frame barely makes it under the 79" limit, I have to custom make a box for it. If I go over the 79" by even 1/2 it will go from costing $75 to ship it to costing $300.
Size and weight makes the difference, so do your homework before you box anything up and you'll be fine.
If you have a FedEx or UPS account and the person at the counter lets something slide, your account will get billed later after it is caught at one of the hubs.