Did you know that USPS will deliver boxes to your door for free? You don't pay for the boxes, you don't pay for the delivery. The only catch is that you have to use them for priority shipping only. But more often than not, priority shipping is just a little cheaper than parcel post when paid online, so we're probably shipping priority anyway.
Let's start with the flat rate stuff first:
The regular Priority Flat Rate Envelope - I use this as often as possible. It's just thin cardboard, but you can shove a lot of stuff in there. I have shipped jeans, heavy sweaters, books, even breakables! If you've got small breakables, put them in a small flat rate box and slip it into this envelope. It will save you a few cents over the small flat rate box, but it will have the extra protection that a box affords. This envelope costs me $4.70 to ship ($4.95 at the post office and $4.90 without the USPS savings program).
The Small Flat Rate Box - I almost never use these. Why? Because they cost $5.00 to ship. The regular flat rate priority envelope costs $4.70 to ship. This box slips perfectly into the envelope. So why would I ever pay $5 to ship it when I can slip it into the envelope for 30 cents less? Good for smaller breakables or smaller items that you don't want flopping around loose in an envelope.
side-loading and top-loading. I almost never use these. Most of my inventory is 2 pounds or less, which means that even shipping clear across the country is only $8.20 for me. These boxes are $10.20 ($10.50 without the USPS savings program, $10.95 at the post office). So unless I'm shipping something heavier than 2 pounds that just happens to fit in these boxes, I don't use them.
top-loading and, more recently, board game shape. I have never used either of these. I got all excited about the board game box until I realized it was a large flat rate box. These boxes cost $13.67 with the USPS savings program, $14.20 without it, and $14.95 at the post office. You'd have to have some large heavy stuff to make these boxes worth it.
Now, on to the ever-confusing Regional Priority Boxes.
side-loading and top-loading. These boxes both ship for the same price as a regular 2 pound priority package. I was using the top-loading box ALL.THE.TIME for shipping shoes. These are the absolute perfect size and shape for most pairs of shoes. But once I signed up for the USPS savings program (this is starting to sound like a sponsored blog post....), the regional box A was actually a few cents more than a regular 2 pound priority package. "A few cents more" sounds petty, but when you ship 200 packages per month, that can add up quickly over a year. So the only time I use these boxes now is for items that are MORE than 2 pounds. The occasional hot rollers fit in here, or a heavy stack of books...
The side-loading one I didn't use too often. I still use it for my Meltonian orders. I can fit a starter kit in here nicely, or 10-12 creams. Then I cut the box down and it slides right into a legal flat rate envelope. Other than that, I have a stack of them assembled and holding my poly mailers.
side-loading and top-loading. I rarely use these, because they ship for the same as a 4 pound package. I rarely ship things that are heavier than 4 pounds, so I wouldn't see any cost savings.
The rest of the boxes I'm going to talk about are just regular priority boxes. They ship at regular priority shipping rates. But some of the box sizes are really convenient for specific things, and like I said - they are free.
priority video box is just a little bit bigger than a small flat rate box. I don't use it much, but I use it to ship up to 6 shoe creams. This box slides nicely into a regular-sized flat rate envelope. This would be a good box to use for small items that you don't want sliding around in an envelope.
12x12x8 is a good one to have on hand. It's good for plates, smaller square board games (like Memory or Yahtzee - just cut the box down shorter), boots, bowls, casseroles, etc....
7x7x6 is another handy one. It's perfect for mugs, but it would also be great for cup/saucers, Christmas ornaments, smaller bowls, knick knacks, or any other smaller item.
So there you go - that should be enough to get you started. Hopefully I've given you a few ideas to think outside the box (so to speak...). Go to the usps website and browse their free shipping supplies. Order a bunch. Get to know the different sizes. You can't beat free shipping boxes!