you can get free shipping supplies from usps.com
I always ship stuff priority mail...buy a small postal scale, put the item/s in it, weigh it then you can enter the exact weight into the ebay form and it will figure out the shipping cost. This way will let you combine shipping easier, too...if they win multiple auctions, you box them all up, weigh it and get the shipping cost. You can figure it out yourself, too (besides ebay doing it for you...probably easier when combining) ... go to postalcalc.usps.gov ... you will enter your zip code, their's and the weight of the package. I always charge the exact amount it will cost me...I figure that the ebay/paypal fees are my responsibility for the convenience of selling stuff. Some people add a dollar or two to make up for the fees.
As a buyer, I get annoyed when shipping costs are alot more than what is actually paid. I've seen people list things really cheap, then charge alot for shipping to make up for the low sale price.
Children's clothing, especially a name brand, sell really well. I think bundling things together is harder, because as a buyer, I may like some of the lot, but not all of it...so then I don't bid. But that can go both ways.
Also, if you take checks...make sure they clear your bank before you ship out. I don't take personal checks...I prefer paypal (even though there's a fee), or a money order. You just never know with people.