It's ok. I know you are new. We will work together through this. Some people will want tracking so you can also put exact shipping then they give you their zip code and you can go to USPS.com type in your zip to theirs, select what type of package such as (large envelope, small box, etc) how much it weighs and it will tell you how much it costs to ship and how much with tracking. If you have a scale that is. If not you'd have to go to the post office.
No lol. Flat rate boxes are outrageous. Better to ship in your own packages. I use a lot of big yellow clasp envelopes, smaller bubble envelopes and medium bubble and regular envelopes.