Definitely recommend the scale. It has made shipping much easier although I still go to the PO as I am paranoid and expect their scales to be something different. It's saved me a lot of money on guessed quotes though.
Regarding shipped items - Ebay does remind you but I have a system I've used for years which is super high tech in terms of keeping track of what to whom and so on....
...A notebook.
Ok, really not being sarcastic now but when I have a bunch of sales on here or anywhere, I write down the ponies and the person for whom they're on hold. If the sale then happens I add the address, and amount...type of shipping to be used...when paid...then when packed...then when posted. And when posted I put a single line through it so I know it's gone but can still see the details if I need it. I also keep the postal receipts as the tracking is on them but I always put tracking on ebay/on paypal/in a Pm to the buyer as soon as I can in case something happens to the receipt.
Once the transaction is over and safely received and buyer is happy, those pages get removed and shredded.
I'm really disorganised so I really break it down to something mundane as I find it easier to keep track of things written on paper than on the PC...
But my real advice here would be to do only as many sales at once as you feel you can handle. Everyone has a different maximum that they can deal with at one go.