How does printing a shipping label work?

[quickdraw_1]

 I have never used the option to print a shipping label with tracking numbers. For those with experience, I just want to know how the process works. all i have ever done is take the package to ship, down to my post office. But my question is, If I print a shipping label with tracking number, how does it get communicated or paired up with the actual carrier? and does ebay charge to print a shipping label?

[fern*wood]

I bet the seconds you saved cutting out your label with wallpaper scissors was lost by going to the post office and dealing with a clerk. 

[quickdraw_1]

Do i need to weigh and measure my package accurately before i take it to Post office? I want to know if i didn't , will they charge me other than what i already paid when putting together my label. therefore being double charged for what i put into it?

[ravenm.-4]

If you're printing a label at home, you need to do that BEFORE printing