I use the flat rate boxes every week. There is a difference in size between the small flat rate box and the small priority box the isn't flat rate. It is just a little bigger, but most people don't notice. Just make sure it says FLAT RATE on the box. I also use the USPS online service to mail my packages. I print the postage at home and save money each time I mail a box as you get a discount. You can schedule a pick up online and your postman will pick your packages up. I leave mine on the front porch for him. Of course the small flat rates will fit in my mailbox. If you haven't tried mailing online, you will love it. You do need a scale which are inexpensive on Amazon. Just get one that weighs up to 50 pounds. No more trips to the post office.