I ship a lot with ebay so I'm in the post office a bunch. There are always people who have no clue how much shipping can cost and end up loosing out on deals because they did not charge correct shipping.
You can calculate shipping from the post office's website, so if you are going to sell something or do a trade, find a box that fits your item, then weigh it and measure it before selling it. Then check the cost to ship to the farthest point in the country from you. Something that may cost me less than $10 ship in my region may cost $75 to ship across the country.
Size matters. Bigly. Going over 12 inches on any dimension can jack up the cost significantly when shipping across the country, but it might have no impact locally.
Flat rate keeps things simple, but not always cost effective. If I have something I know is heavy and will fit in a flat rate box, I might sell it with free shipping and factor in the flat rate price in the listing amount, but if someone relatively close buys it, I can ship it for $10 less with normal priority mail. Most of the time, though, I'll know my dimensions and weight in advance and choose calculated shipping and have the buyer pay the calculated price. I know how much I want to get out of an item and if someone across the country wants to pay 2x-7x more for shipping then someone close, then fine.
First Class is wonderful. Buying online shipping labels I know I can ship a package weighing 4oz or less for under $3.50. If I'm selling one or two metal minis at a time or a bunch plastic minis, I have 4x4x4 inch boxes that keep weight down and are strong enough to keep the stuff safe.
Also, shipping things nowadays is really wonky. I've had priority mail packages take crazy amounts of time to ship compared to the way things were before Covid. It's weird and doesn't follow any pattern I've been able to discern.