Boxes are expensive to buy and hard to find in the right proportions. Boxes for flat screen TV's or other electronics might work. Most large stores compress their cardboard, making boxes even more scarce. You might find small businesses in your area that get product in boxes you can use. Smaller operations are usually happy to get rid of the stuff. I wouldn't dream of selling a pack before I had an appropriate box; otherwise, it is a scramble and you lose in time and dollars. Having a vendor box an item would seem very expensive to me.
Sometimes you can nest two smaller boxes to make one large one. You want some good overlap and more tape is better :)
Shipping varies with the weight, size and destination. You may find "holes" in one or another carrier's fee. Years ago I found that FedEX Ground used oversize dimensions just a few inches larger than UPS and it was a large savings on the item I was shipping. It pays to compare.
UPS and FedEx Ground and USPS Parcel Post are the real choices. You can get pricing on line. I did find that Office Depot had good prices on UPS services vs The UPS Store and other vendors. Again, it pays to shop. You need that box to get accurate dimensions and weights. You can certainly sell your item "plus shipping" and give quotes per service and destination.