Any rules for eBay shipping charges?

I have been selling on ebay for 8 yrs, and for the longest offering free shipping worked well for me. I would estimate the cost of shipping and just add it into the price of the item. However, I began to loose money and I started charging shipping cost and I sorta saw a drop in sales, but then again, it might have been one of my unproductive months. Personally, as a shopper I look at only items with free shipping, even though I know that the seller is adding it onto the cost of the item..how dumb of me is that ? lol. Oh one other thing, depending on what you are selling would also make a difference in what you should charge. I sold jewelry and small items, BUT if I sold a lot of clothes or books, I automatically used priority shipping because of the weight. Unless it was a huge lot of clothes then buyers seem to not want to pay that price. The best thing you could do is test the shipping out on different sizes and weights of items. I wish you the best of luck in your ebay adventures.