When buying different variations from the same or similar listings, the lowest shipping cost is obtained by buying the total number of units wanted from 'One Variation' and then leaving a message within your order defining what items you want to receive and the number of each item you want to receive.   The number of items requested cannot exceed the number shown as being in stock.  This stops from restarting the total weight summation for each variation. 

The message link is on the same page where you select method of payment.USPS postage cost is based on total weight and zip code. The cost breakdown is shown before payment is required and the discount for multiple purchases is shown as a separte credit in the list.