USPS International rates

[Arunachala]

We ship books internationally and generally use First class international mail. The large envelope(12x15x3/4") appears to be the only option available with First class. When the book order does not fit we use (12x12x12) box but that is available only in post office not online. It costs more so we lose money. The next option is priority mail which costs a lot more. I wonder what other stores are doing to get around this problem. I added extra handing charge for International order. Is there a better way to address this issue.