Ok, your item sold and you received payment from your buyer, and now is the time to deliver on your promises by shipping the item in a timely manner.
I personally try to ship the same if not next day from payment, even though all my listings state 3 business days. Why do I not list 1 day handling you may ask? Well I look at it as a case of under promise but over deliver. This means I set a lower expectation in my buyers mind, yet over deliver when they receive the item sooner than they had expected.
This has resulted in countless amounts of positive feedback stating fast shipping, and actually had one referred to it as super sonic. I do own my own shipping scale so that makes processing my shipments quicker, they print directly from the paypal transaction, and cuts down on trips to the post office.
If you do not have a shipping scale, a trip to the post office will be required. If you have a tight schedule, and only ship on certain days, you will want to make sure you document that in your listing. Also I highly recommend including a tracking # with EVERY shipment. This helps protect you the seller, especially should one claim that they never received the item. Its kind of hard for a buyer to argue that they never received your item, when the tracking details shows in black and white when it was delivered.