Make shipping cost from more than one carrier be seen simultaneously.
Please consider making at least two shipping carriers rates be seen at the one a single screen so it isn't necessary to toggle between carriers to compare their costs.
A good example of how this works is how Amazon does it. They show a "laundry list" of the available carriers all at the same time which makes selecting the best carrier to fit the need very easy and simple.
Josh Trampoline Pro commented
Either this or an automated rule that picks the least expensive carrier (pre-selected based on package size, weight, etc) would be very helpful