1) Sorry, it's not possible currently. We calculate the shipping based on USPS standard where available and if it's not available (for example, USPS standard is not available for packages over 13 ounces in U.S) we use USPS priority.
2) Yes, please see "cart customization" in our help page.
3) We provide a CSV download, but for QuickBooks, you will get data from PayPal.
4) Sorry, we do not provide packing slips :(.