Invest in a postage scale, you can pick them up at the post office, target, walmart, etc. If you are shipping heavy items, like I do, I use a regular floor scale (bathroom scale) and get an approximate weight in lbs.
From what some other team members have said, the USPS current prices for international shipping are now in line with most other countries. Your international buyers will not likely be deterred by the new higher shipping prices. Though it doesn't hurt to check on pricing.
