Honestly, postage rates will vary so much by destination, size and shape of the package, and options like registered mail that it's doubtful anybody but the post office can answer this for you.
You might consider:
a) Letting customers have whatever shipping for which they're willing to pay, and charge accordingly -- this removes the accounting and scraping off of your list of things to do.
b) On the other end of that scale, you might consider selling higher-end items into which you can bundle the price of shipping, then offer free shipping as a carrot to buyers.
c) Offer upsells on orders to increase your profitability-per-package. Get a feel for how many of your 'core item' you can cram into a single shipment without dramatically increasing the shipping cost, and offer discounts on those additional items -- it can dramatially affect your profit per order. Alternately, if your 'core item' is large, some small accessories or complementing items which can be added to the shipment can help offset the shipping costs in the same way that offering quantity discounts can.