[FIX] Allow carriers to have a normal price of 0.00

Currently, if you define a carrier without advanced price rules and a normal price of 0.00, and no "Free if more than amount" price, you get an empty grid and then errors when trying to create invoices for delivery orders.

This fix skips the special case for (Normal Price == 0 and "Free If more than amount" is False) and simply generates a grid with the normal price as the last entry.

The help messages for the different fields are also clarified, to reflect the fact that both "Normal Price" and "Free if more than amount" are ignored when advanced price rules are selected.