I have all but quit selling on evil-bay entirely for a number of reasons, but postage costs in general have gone out of sight in the last few years with the USPS attempting to cover its increasing losses, etc.
If I were a business, I could afford to invest in some boxes, however, I'm not, so I use the USPS Priority Mail boxes because they are free. While that forces me to use priority mail, in a sense I don't mind because I've had better luck as both a seller and a buyer using priority mail.
I also do not sell internationally anymore. After seeing a number of friends get burned by unscrupulous buyers who buy an item, then claim they didn't get it and demand a refund, etc. it just became not worth it. Add to that the hassle of customs forms, etc., and I just decided to opt out. Sadly, for now on it's domestic only for this kid.
Warren