I don't know if this helps you or not (since you're not familiar with the US postal system), but this is information I copied from the USPS website regarding current postage rates for letters, large envelopes, and packages. (It's a word document; let me know if you don't have the right software to open it, and I'll do a copy and paste in another post).
2013 Postage Rates.doc
Letters: If you're mailing a Christmas card or simple letter that isn't too bulky or thick, weighs less than an ounce, and is rectangular in shape, most likely you won't have to pay more than the minimum first class letter rate which right now is 46 cents. The price goes up if your card weighs more, is bulky or thick, or is a different shape such as square (see attachment for size regulations).
A postcard is currently 33 cents.
(FYI--if/when you go to a U.S. post office: if you buy a bunch of 46 cent stamps, make sure they are "Forever" stamps. This means that even if the price of postage goes up, that stamp will still cover the cost of a one ounce first class letter. (And they are talking about postage rates rising again in January 2014.) Just in case you come back next year. ;))
Large envelopes: If any of your cards are larger than 6-1/8 by 11-1/2 inches, but still flat and not bulky (less than 3/4 inch thick), this is the rate you would be charged to mail them within the U.S.
Your calendars may fall into this category but only if they are smaller than 12 inches by 15 inches and once packaged are less than 3/4 inch think. (FYI--Out of curiosity I measured my wall calendar and it's 11.5 x 13 inches.)
Packages: Attachment lists the cost per ounce. The post office will not only weigh your package, but take its measurements and charge accordingly, meaning that the larger or more unusual shaped your package, and the heavier it is, the more expensive it will be to mail it. As for the zones, the clerk will plug in the recipient's "zip code" as part of the process. This is the 5 digit (or 5 digit + 4 digits if you have them) code that follows the state code.
If you're sending just a few chocolate bars to a person, I would echo the other poster who recommended buying some padded envelopes once you are here. You can get them at just about any grocery store, drug store, discount store (Target, Walmart, KMart), office supply store (Staples, Office Max, Office Depot), even the dollar stores. Depending on how many candy bars you put in each one, they'll probably fall under either the large envelope category (for maybe one or 2 candy bars, if they're laid flat), or a package (for several candy bars stacked on top of each other). There are usually 2 different sizes of padded envelopes available: 6x9 inches or 9x11 inches.
For heavier gifts or bulky items, I like the
priority mail flat rate boxes. As mentioned above, these boxes are available free at any U.S. post office and they come in several different sizes. The cost is set by the size of the box regardless of what it weighs, as long as it weighs 70 pounds or less and the box will close on its own. :p They now come with free insurance, tracking, and are usually delivered in 1-3 days depending on how far they're going. And the boxes are sturdy. You'll still want/need bubble wrap or other cushioning material if you're sending nice gifts, but at least you don't have to pay for the box. And if you're mailing something that weighs a lot, they're definitely the best option.
Sizes and prices of the flat rate boxes are (I'm copying this information from USPS website):
Large: Priority Mail� Large Flat Rate Box: 2 shapes: 23-11/16" x 11-3/4" x 3" or 12" x 12" x 5-1/2" . Cost $16.85.
Medium: Priority Mail� Medium Flat Rate Box. 2 shapes: 13-5/8" x 11-7/8" x 3-3/8" or 11" x 8-1/2" x 5-1/2". Cost $12.35.
Small: Priority Mail� Small Flat Rate Box. 8-5/8" x 5-3/8" x 1-5/8". Cost $5.80. (This is about the size of the old VHS tapes, if that helps.) I use this size box all the time to send little handcrafted things to people. If your handmade cards have a lot of embellishments on them, and if you plan to include several candy bars or other small gifts with that card, I think this is a good option for you. It will certainly be less hassle for someone unfamiliar with U.S. stores and post offices.
Hope this helps. Enjoy your trip!
US Postage Rates
