You could, but I would highly NOT recommend this. I go to the post office in person and hand them to the clerk personally. It's safer that way.
Yes and no. You could print the label today, and if the "ship" date is today, you could physically ship the item tomorrow or the next following day since it was already paid. However, you cannot ship today if you paid shipping for tomorrow. For example, you do the online label thing but say to "activate" tomorrow. You have to ship tomorrow if you did this, because if you said to "activate" tomorrow and you ship today, it's "not registered". Nothing may happen but I would be "safe than sorry".2: eBay asks for a ship date for your item before you even drop it off. If I print a label today, and then ship it a few days later, would this be a problem? Do you have to ship your item the exact same day you print your label?
When I ship items, I prepare a label for the same day I have the package prepared and then ship the items the next following day. If I were to ship items on Monday (the 21st) I would print a label stating I would ship on the 20th. This way, when I ship on the 21st, it's already "active".