Yes, 714ricki advises what is really the only "solution". I include in my listings the following, I got this idea from another seller, include it in my listings and anyone is free to use it. Bay does not do well with combined shipping calculations for the reasons shown below. It's not perfect, but....
Combined Shipping: To see the combined shipping discount, please use the "Add to Cart" button instead of the "Buy it Now" button to place all desired items in your cart. Prior to checkout, make sure the ship methods for every item are the same and are NOT set to First Class Mail as this method will not combine. We recommend trying different shipping options as the combined discount for each method will vary. Once checkout is initiated and before you pay for the order, if applicable, you will see the discount applied to your order's total. Combined Shipping does not always work perfectly, eBay combines the item weights but does not take into account the possibility that a larger package may be needed. Seller reserves the right to adjust or change shipping methods to avoid incurring additional seller-paid shipping charges.
