How accurate is the shipping wizard on ebay?

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Normally when I list my items, I guess at how much the shipping will be, and usually add some money to that just in case I under-estimate. Most times, I may even use a flat rate box just to make sure i charge the customer the right amount and don't lose money on postage. Anyway, for my recent listings on some of them I've used the shipping wizard. I put in the weight and the package dimensions, and I see that it's now listing a price for shipping based on the buyer's zip code. Anyway, has anyone used that? Is it accurate? If you put in the correct weight, and dimensions, will it charge the customer the right amount? I"m so leary of using it, but wanted to make it better for my buyer to get accurate rates, instead of my guess on how much it will cost.