The web site computes the shipping charge for each transaction. Many times this amount will exceed what I actually pay for the shipping. I will refund your credit card the difference between the charged amount and the actual amount, unless it's like 2 dollars or so. Then the transactions fees with card processor make that not practical.
Because models can be large but not heavy odd shaped kits like a ship, can cost more just because the total number of the inches of the package measurements.
Remember, I want you to a happy customer, I want you to come back and shop again at my web store.