Both those files are always there, they are core files. The question is, whether they are the original Zen Cart core files or the Big Royal Mail modified files.

You posed 2 problems with your installation, one of the non appearance of prices for certain shipping combinations, and one of Constants appearing instead of the text they represent.
My reply about shopping_cart.php was directed at the second problem.

Regarding the first, you mention a shipping weight of 740 grams. Do you mean a shipping weight of 0.740kg, or a shipping weight of 740 shipping weight units, hopefully grams and not kilograms?
Could you list from Admin-Configuration-Shipping-Packaging the values of

Maximum package weight
package tare small to medium
Larger packages tare
Number of boxes

You could also set the shipping estimator there to 2, so that as you add items to the cart, you can view the changes.