I know the shipping aspect can be intimidating at first and can seem rather daunting. I actually go back and forth between shipping through a local shipping place and doing it myself, it depends on the time I have and the size of the painting. I've built a repertoire with the place I ship through over the past 4 years or so and finally took the plunge a few months back and it's helped to have someone who can wrap up and take care of the really big ones instead of me fooling and fussing with it at home. When I do it I buy frame boxes and bubble wrap from them, there's a bunch of sizes to choose from and I buy a good amount at a time. I have an account with FedEX and USPS.com and I have them pick the packages up. The accounts were easy to set up and it's really convenient. You can also print shipping labels through paypal - hee's there help explanation on their site - https://www.paypal.com/
It's really quite simple. You do need a scale - I got mine at Walmart for 10 bucks.
Larger paintings should really go through FedEX because of the cost and the shipping "zones". 16x20 I'd send through FedEX. 11x14 can go just fine through the postal service.
For all of my small shipments (anything up to 12x12 or so) I use the free boxes you can get through the post office. You can order some online for free - they are for Priority shipments though so if you plan on sending your paintings first class you can't use the free boxes. I always send Priority when I use USPS because it looks more professional and is faster for the most part. Here's a link to order free Priority boxes - http://shop.usps.com/webapp/
The place I go to for my other boxes orders theirs from uline and sells them to me at cost. It's good to start a relationship with a local place cause there can definitely be perks! Take a day and shop around. A really large box for me costs 16.00 - that's for a 36x46 box, unfortunately sometimes you gotta cut um down since they don't always have the size you need) I have heard some people go to Michaels and get their boxes on garbage day, but you have to be there at the right time - they wouldn't hold them for me and it was like 20 miles for me so I just broke down and bought them outright. But that is an option.
When I am wrapping it myself I wrap the painting in plastic and tape it to secure moisture from compromising the painting. Then a layer of bubble wrap is tightly wrapped around and taped. Another layer of bubble wrap is then wrapped around the first, bubble to bubble, creating a "pillow" that is extremely effective in securing the painting from damage. The pillow is then placed in a sturdy mirror box for shipment with more bubble wrap or paper if needed.
Please copy and paste this URL in your browser to see how these "pillows" look just before shipment - http://tinyurl.com/5ws4ah
Note that with international shipping, to most countries the largest stretched canvas you can send is 22x28 through the postal service. The postal service has strict dimensional guidelines - length+girth (a tape measure wrapped around the middle of the box gives you the girth) can't be any larger than 79 inches. This includes Australia, a popular shipping destination. For places with the 79 inch cut off I offer taking the painting off the stretchers and rolling it in a tube. This doesn't always work though. I can't do this with gallery wrapped canvas, only with canvas that has staples on the back - I can take staples out of the canvas. Can't rip it from that groove the higher end canvases have, and I won't cut the canvas from the stretchers. It's best to advise your patrons of these things so they are aware. That's why in my shop I only show US and Canadian shipping prices for larger works. Canada has a 108 inch cut off, so pieces up to 24x36 can go through USPS. Any larger and it has to be sent through FedEX or UPS - which for an international destination can be a couple hundred easy. If a patron is willing to pay the actual shipping cost then by all means. But it really is exorbitant!
It looks like a lot to take in, and initially it is - but after doing it a while you'll become a pro and it will be second nature :) Best of luck!