It's often easier and more accurate to leave exact shipping-cost calculations -- both to you and your customer -- to the carrier you use. The major carriers will calculate shipping costs for you for free, and most major ecommerce vendors allow you to integrate those costs into your store's checkout section.
On the other hand, you may want to consider offering free shipping, at least for orders over a certain amount. If your direct competitors provide free shipping, Peterson suggests that you consider it, too, if you can afford it.
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