You need a scale. But I think that Amazon.com has the weights of many of its books in the description, so you may be able to use that weight, plus the weight of your packaging (which maybe you could use 1 consistent packaging, say a specific padded envelope, every time, to make this easy). Using the weight and your zip code and the destination zip code, you can get the estimated shipping online from USPS.com. Since all books weigh different amounts, this will be your best bet, in my opinion.