It's not exactly a well known fact, but you can, in fact, ship most things without leaving the comfort of home. There are limits of course, but knowing how to navigate the USPS system and get the most out of it can streamline the shipping process!
As a note, you DO need to have a USPS account and a printer for most of these awesome things to work. Yes, it's one more account to keep track of but I promise - this one is really useful.
USPS supplies free flat-rate Priority boxes, customs forms, and some other supplies at most post offices, which is great news in and of itself. Did you know that you can have those and other (more useful) sizes shipped to your house - completely free? It's awesome. I love the 7x7x6 sized box for mailing out finished saddles and Stablemate scale resins.
You need an account to order the supplies, and you have to add them to a cart (which sounds scary at first), but no payment changes hands. The boxes are shipped in multiples of 10, which is great for me - I order them every couple of months and I'm set for a while! The hitch is that these free boxes are Priority/Priority Express only. There are no options to snag free boxes from USPS for anything else. Since USPS doesn't have to give me anything, I'll take my Priority boxes and be happy about it. Bubble mailers can be purchased (in bulk) for pennies apiece on Amazon and there are other retailers - online and not - that offer boxes.
Link to free supplies here.
You'll need a printer and a scale for all of the postage options listed here. Digital postal scales can be purchased quite economically from Amazon, but you can also use a kitchen scale.
Without an account, you can estimate postage costs on the USPS website. Even if you plan to purchase postage through another service (like eBay, PayPal, or Etsy), this is a fantastic way to get a quote for a potential sale. Input your ZIP code, the recipient's ZIP code, and the rest of the info USPS asks for and you're well on your way to getting your quote. You can even price insurance and other additional options. Here's the link to the postage calculator.
USPS usually gives you quotes from over-night shipping to media mail, with two pricing columns: one for purchasing online and one for purchasing in person at the post office. Make sure you read the options carefully and understand their limitations before you pass the quote on to your customer. You can do international quotes, too, but be prepared for sticker shock.
You can purchase postage online through USPS Click-N-Ship. It's pretty painless and usually cheaper than going to the post office, but you will need an account. If I remember correctly, you don't have to attach any bank or credit card information; you can pay for shipping via PayPal. Here's a small hitch: in my experience, you can only purchase Priority shipping (or more expensive options) through Click-N-Ship; like the free boxes, there are no options for purchasing First Class, Retail Ground, or Media Mail postage. If you took a money order (or a check, or cash) as payment, this is the cheapest way to purchase shipping - if you had planned to ship Priority to begin with.
If you took payment through PayPal, you have even more options - as long as you were paid with the Goods & Services option. There are many good reasons to not accept Friends & Family payments, but the only one that applies to this post is the ability to print a shipping label through PayPal's interface. Once payment hits your account, you can click on the payment details, scroll to the bottom of the page, and locate the [Print Shipping Label] option. Once clicked on, it redirects you to a new page where you can enter the specifics of your shipment, including services like insurance and signature confirmation.
There are two areas where PayPal rises above USPS. PayPal shipping is usually cheaper than a Click-N-Ship quote; there's likely a bulk deal between PayPal and USPS, but I don't know specifics. I just know that shipping is usually cheaper, up to a dollar or two (but not always that much). The second and very important advantage to shipping via PayPal's interface is that you have the option to select more than just Priority! Media Mail and up are options through this interface, and it's really handy sometimes.
International shipping: through PayPal, at least, you can fill out customs information online. The customs label is printed as part of your shipping label, which makes life really, really easy.
Quick tip: for either service, you'll need to weigh your package and measure the dimensions of your box. You are limited to entering weight by pounds/ounces and measurements by full inches, so to make sure your package isn't returned by the USPS for insufficient postage (or that your buyer isn't stuck footing the bill), always round up to the nearest ounce or inch.
This little trick doesn't require an account, a printer, or a scale, but probably isn't much use if you haven't printed postage at home. USPS offers free pick up of packages (within some limitations) during your normal mail delivery. You must put in the pick up request before 2 a.m. the day of pick-up (you can also edit it up until that time). If you live in an apartment building or don't have a secure location to leave packages while you're at work, this might not be a good solution for you. I live in a house in a pretty safe neighborhood (plus I work from home), so I don't have a problem leaving packages on the front porch.
You can schedule a pick up here and read more about parcel pick up here.