That's correct - once it's scanned that's it. The label is used and paid for.
If the weight was wrong the clerk should have sent it on its way because USPS has something called APV (automatic package verification) which is a system where packages are weighed, measured and photographed as they pass through sorting machines and the system will automatically apply cost adjustments to your account when discrepancies are found.
If the clerk was not willing to let the APV system take care of it she should have checked the weight before scanning the label. The clerk messed up and you are paying the price.
You can take the complaint to your local post master and see if they can do anything to help, but they probably can't do much other than maybe talk to that clerk to hopefully avoid repeat situations in the future.
Also - if you don't have a shipping scale you should get one. They are inexpensive and well worth it.
I'll be honest: I'm a bit concerned for you. Broken item sold for parts, expensive shipping, expensive item. I worry the buyer will file a claim and I worry about how it was packed and whether you'll see cost adjustments from FedEx. If you entered wrong weight/dimensions on the label you can get slammed with a big shipping cost adjustment from FedEx in the next several weeks.
