I used to be an eBay Powerseller, so I was always mindful of margins, expenses and time. I'm going to approach your question from the angle of shipping in general. I assume you're also inquiring about safe shipping for plants specifically. I'll respond with my approach in another post, and i'm sure other members will chime in as well.
First, I highly recommending printing your own shipping labels, rather than going directly to the post office. Not only does it save you the time, it also saves you money b/c you pay a discounted rate on all shipping, whether its 1st class, Priority, Media or whatever. For example:
Shipping costs if you print your own label:
Shipping costs if you purchase at the Post Office:
1. If you want to ship Priority (2-3 day shipping), get flat rate boxes from the post office or have them dropped off at your house for free: https://store.usps.com/store/results/bu ... -supplies/
2. Log into your PayPal account.
3. Go to this link to print your shipping label: http://www.paypal.com/shiplabel/create/
4. Go here to request a free USPS package pick-up: https://tools.usps.com/go/ScheduleAPick ... put.action
Print your shipping label, stick it on the box, schedule the pickup, and set the box on your porch. The post office will do the rest. You never have to leave your house.