I shipped seven different packages back to the US and decided to post the different shipping options here in the hopes that someone may find it useful. I had the luck of being helped by a very nice guy at La Poste back in April when I was sending a present back to the US, so when it came time to ship my stuff in July I was prepared with my options. I'll include prices for other non-EU anglophone countries as well.
Option One: If You're Sending Books (to the US, maybe others?)
Boxes of books up to 4 kg or 8.8 lbs (although I think they'll take up to 5kg because my box came in at 4.2 kg) have a special rate in order to "facilitate cultural exchanges" (that's what the guy told me). Of course I can't find an actual rate online, but the package I sent that was 4.2 kg only cost me about 13 euros. It's a great deal, and I definitely recommend sending your books separately in order to save some money.
Option Two: Sending a Small Envelope (all non-EU countries)
You can send up to 1 kg (2.2 lbs) in small envelopes that come with a little bubble wrap to help cushion things inside. They only cost 14 or 15 euros, so this is also a good deal if you're sending something small and not too fragile. The envelope is called Chrono Express and it arrives in the US in a week. It's cheaper than making a box of things that weigh 1 kg. Sending a 1 kg box to the US, Canada and South Africa costs almost 17 euros, to Australia, New Zealand or India it would cost 21.50. It isn't a huge saving to the US, Canada, and SA, but every few euros helps, right?
There is also a similar Chrono post box that you can send for overnight delivery but that costs about 90 euros, so I only recommend that in case of emergency.
Option Three: Make Your Own Box (all non-EU countries, different rates apply)
You can pick up any box you have lying around or can scrounge from your schools or local businesses, pack it, stick on an address, then take it to the post office, fill out customs forms, and pay for it based on the kilo. If you're sending between 2-5 kg (4.4 - 11 lbs) that isn't only books or over 7 kg (15.4 lbs), this is probably your best option. 3 kg (6.6 lbs) costs 25.20 to the US, Canada and SA and 36.90 to Australia, India or NZ. If you send 10 kg you spend 75.60 to the US etc and 93.60 to NZ etc.
Those are the "economique" prices, there is also an express price but again, it's extremely expensive and not worth it unless it's urgent.
Rates can be found here.
Option Four: Colissimo International (all non-EU countries, same rate)
In my opinion this is probably your best option for non-books. You can buy flat-rate international boxes from your local La Poste, put your stuff inside, fill out some customs forms, stick on an address, and then drop it back off at the post office. A large box that holds 5 kg costs 35.50, or extra large costs 41 euros and holds up to 7 kg. The weight limits don't really count because no one weighed my packages, so as long as you aren't sending 15 kg, I wouldn't worry about it. These rates are better than you can get making your own package, especially for Australia, NZ and India. The downside is that the boxes are sort of unsturdy, I put a lot of extra tape on them, but all of my stuff arrived in good shape and the boxes didn't break, they just bent a bit, so it probably isn't a good option for anything fragile. They arrive in about a week.
Hope someone finds this helpful!