To add multiple boxes to the Package Dimensions field, measure all boxes and add together the length of each of the boxes, the width of each of the boxes and the height of each of the boxes.

For example, if there are two boxes that measure 1'L x 1'W x 1'H and 2'L x 2'W x 2'H, the dimensions entered would be 3'L x 3'W x 3'H.