You need to keep your stock at your place and pack them when someone orders one.  If you anticipate lots of one-item sales then you can spend your time pre-packing them while watching TV at night, so you save time when someone buys one.  Just slap on the address label and go to the post office.

You can even get a statement form from the post office to apply your own tracking stickers and state how many you're sending within different weight ranges, and it's all done within a single transaction.  Very quick.

You don't have to go to the post office every time you make a sale.  You go two or three times a week and send them in batches.  Adjust your "handling time" to match (I have mine set at 5 days to account for postal delays also) but always go within 2 business days with most sales.  I time my post office visits with when I go grocery shopping during my lunch break, so I am maximising my time efficiency.