If you have a scale, it will help you in determining shipping type and class. Shipping rates are changing in April, by the way.
Sometimes, you will have an item (stickers, gift card, etc.) that could fit into a regular # 10 envelope (standard letter size envelope- business letters are sent this way). In this case, you can use a regular stamp: 44 cents- as long as it is less than 3.3 ounces.
First Class mail is anything under 13 ounces in weight. A “Flat” is exactly that (scrapbook paper, photo paper). A “Parcel” is a package that is not flat, thicker. In terms of weight for a package, this is the cheapest way to go. In this case, I usually reuse the shipping clasp envelopes. The envelopes with the bubble wrap built in are not recyclable so the best way to deal with them is to reuse them, and encourage others to do the same. I think that Tyvek envelopes are recyclable but they have to be sent to a company.
Media Mail is generally Videotapes, Cassettes, CDs, and Books. It may take longer but it is cheaper. For a small book, the average price is $2.38 (starting). If it’s lighter (softcover), you might want to ship it First Class- could be cheaper. I belong to several book swap sites (one ended), and the average price for a regular sized book is about $2.77. I’ve also sold CDs to music companies (www.secondspin.com, and shipped a considerable amount of CDs (more than 50) in a large box, for about $6.80.
USPS Priority Flat Rate boxes (and flat rate envelopes) work well for heavy or bulky items. There are several sizes: small, (2) medium- long, medium-thick and short, and large. I usually do a test run to see what will fit my needs best (since I usually do multiple items in a swap and sometimes include freebies- lots of swappers also do this too, in order to save on shipping). Sometimes, I get lucky and the items fit into a flat rate envelope (the same price as the flat rate small box)- I usually reinforce the edges with clear packing tape. For larger swaps, I usually use the long flat rate medium box. There is no weight limit on the Flat Rate boxes, unless you ship overseas (for this reason, I have limited my swaps to the US). If you have an online account with USPS, the rates are slightly cheaper (commercial based pricing), and DC (delivery confirmation) is free. You will need a printer to print out the shipping label. I also reuse the USPS boxes and envelopes, too- also, recyclable, since it is cardboard.
A note about “Parcel Post”, it takes longer to ship, and it is usually a few cents less than shipping “Priority.” I’ve received a few packages this way, but I usually don’t ship this way.
Some swappers use other means of shipping, like UPS- I am not sure about what the rates are, though. The majority of swappers use USPS.