Most economical carrier for shipping steel plates ranging from 10 to 30lbs each?

I am starting to ship more and more flat steel plates domestically in the US in sizes ranging from 6x6 inches to 24x24 inches that weigh anywhere from 10 to 30lbs each. Anyone have experience in what the most economical shipping carrier would be to go with for these types of items? Or any tips on how to get the best rates for shipping these?