So you sold something on eBay, now what?
This question is often asked in the various groups & forums, so here's the simplest answer:
Pack it up and ship it.
The longer more detailed answer:
First of all, don't be afraid of the shipping fees and nuances of shipping on eBay. It's not as hard as some people make it out to be and shouldn't dissuade you or run you off from selling on eBay.
When you set up your listing on eBay, you had to choose a shipping option. There a multiple options when picking out how and which service to use on eBay. One option is "free shipping" which means you the seller are going to cover the cost of shipping the item. A lot of sellers offer "free shipping" then pad their sales price by a few bucks to offset the shipping cost. I offer free shipping on items that are lightweight and priced at $50.00 or higher. I also offer free shipping on items that are $100.00 or over regardless of weight.
Ebay has a long-standing relationship and negotiated deal with the USPS, so you'll find that using that pricing structure is more than adequate. What will happen when you choose to ship using USPS through eBay (and not free shipping, so the buyer is paying) is that eBay will calculate the price of shipping and charge the retail rate to the buyer. The buyer will pay for the price of the item, the sales tax, and the shipping fee. You'll receive a notice that the buyer has paid then you click on "ship the item" which will take you to the shipping page. You'll have already told eBay how much the item weighs when you set up the listing, so now you'll pick which USPS service to use - first class for under items weight 15.9 oz, priority for 1 lb and over, priority flat rate if applicable, and sometimes media mail. Based on what the buyer has paid, pick the shipping option that is most appropriate. Your price for shipping will be less than what the buyer paid as a perk for using eBay/USPS shipping - meaning, you get a discounted shipping rate.
Currently, eBay will allow you to ship via UPS as well and I recommend this option for large or bulky items, or if it's really heavy and won't fit into a flat rate box.
If you decide to not bother with shipping using any services eBay offers, there are a multitude of third party shipping services you can choose. There are pros and cons to each, so do a little digging before picking a service. Also, you will have to choose free shipping to be able to ship third party on eBay (and on most platforms except Poshmark). To my knowledge, all these online shipping services are free to use and have an account with except for Stamps. They all offer and boast they have the cheapest shipping rates available, so again, compare for yourself.
I find it easiest to use the services and rates offered by eBay for shipping. If you use an external shipping service, you'll have to upload the tracking information manually to notify eBay and the buyer that you have shipped their item. If you use shipping through eBay, that is all done for you automatically.
Now that we've gone through all that - you are allowed to use any kind of packaging you'd like to ship your item. BUT, do be mindful about what kind of envelope or box you are using. USPS offers non-flat rate priority boxes on their website, free of charge and shipped to you. In post offices across the country, you can pick up flat rate priority shipping boxes as well as priority mail tyvek envelopes (that are not flat rate). You can use any kind of plain old box and ship it first class or priority. You will not be able to use your own box to ship priority flat rate. You can use any kind of poly mailer bag or padded envelope as well. You can also use product boxes to ship via USPS - some buyers and sellers have said they've used or received items in cereal boxes, cracker boxes, diaper boxes, and so on. I've used my protein shake boxes - they're very sturdy! You cannot use product boxes when shipping via FedEx or UPS unless you cover the box with brown or white paper.
A few tips to make it easy and painless to ship on eBay:
For additional tips on how to pack up your item and priority mail versus express mail, see Shipping on Poshmark.