The only way to avoid issues is to use a simple postage method. It works well for items that have similar postage prices. If you just charge a flat postage fee then an extra amount for each item purchased after that. 

So an example would be if you charge $10 postage for the first item then an extra $2 for each item after that. You increase your item price to allow for the reduced postage price and you make it simpler for the buyer.

This works well for clothing sellers who have enough items where buyers will want to purchase more. Does not work for book sellers or sellers of large items.