TL;DR The best solution is just to print your labels ONLINE with eBay, PayPal or USPS.com (depending on the origin of the transaction), which include FREE tracking and discounted postage rates. Also never have to wait on a line at the PO again.
This only relates to First Class Parcel and Media Mail packages. Priority and Standard mail classes both come with valid (real) tracking included in the postage price, regardless of where you buy it (online or retail).
Again, as I wrote in my prior post, even if you don't buy the tracking on your FCPackage, it will still be assigned a PID# (Package Identification Number). While similar to a tracking number, it doesn't include the ability to view any of the online tracking scans, including the one at delivery. Since that is the only thing that eBay/PayPal accept as valid proof of delivery, it's pretty useless for any online commerce transactions you're involved in as the seller and when payment is via PayPal. I have no idea how the other payment processing services work (ProPay, Google, etc).
To get around it, you either pay the $1.05 at the retail counter/kiosk, or, the much smarter alternative is to print your labels online, where the tracking is included FREE (Media Mail $.23).
Obviously, if it's an eBay transaction, you should print the postage right from your MyeBay. If it's off-eBay, and you were paid via PayPal, then you can print it directly from PayPal. For both of these, the information will upload automatically, and you'll be using the address supplied on the PayPal transaction. No chance for making a mistake while copying the address.
If there wasn't a PayPal payment involved, but you want/need to send a package (personal stuff, Xmas presents, whatever), use this link to get to PayPal's postage label printing interface: www.PayPal.com/shipnow. Bookmark it because you can't get to it any other way. The only unfortunate drawback is that it doesn't have the newer Priority box and envelopes options available (padded/regional). They just never got around to updating it. If you do need to use those, you can print a label at USPS.com (but over there, you can't print FCPackage or MediaMail labels). Because, really, why make it easy for everyone? ;)