The website of the United States Postal Service provides a calculator for determining international shipping rates. According to this calculator, as of June 2014, the cost to ship a large flat rate box to Columbia is either $76.50 or $80.50, depending on whether or not the postage is purchased online.
As of June 2014, international flat rate boxes have a weight limit of 20 pounds. If the clothing items weigh more than this, they need to be shipped either as separate shipments or using a method other than flat rate mailing boxes. Shipping rates change frequently and other factors can affect the final shipping cost, such as whether you choose to purchase insurance and the value of the item or items being shipped. For the most accurate shipping cost, always either check the USPS website or visit a USPS office in person.