Get a sturdy cardboard box that can accommodate your pumpkin. Wrap the pumpkins in some bubble wrap and place them inside the box. Fill all sides of the box with packing materials to prevent the pumpkin from rolling around. You can ship together smaller ones but larger pumpkins are best boxed separately.
Make sure that the pumpkin/s are secured in its/their places before closing and sealing the box with packaging tape.
Address and label the package and bring to your local post office or shipping company.
Note that large pumpkins can get quite heavy so be prepared to pay extra charges. Also, make sure to use heavy-duty boxes for such pumpkins.