Shipping smaller things:

Small devices

• 3DS, Switch, Game Cartridge, etc. 
• Should be around $10 to ship.
• Priority Flat Rate Small box is usually the optimal price

Medium Devices

• A Steam Deck, some controllers, a GameCube, etc.
• Should be around $15-$30 to ship
• Priority Flat Rate Medium or Large are sometimes optimal
• Usually it's cheaper to go UPS Ground or USPS Ground Advantage
• For Ground, try to be fussy about the exact weight and size, carriers certainly are

Full Size Consoles

• Probably don't ship really big or expensive consoles: PlayStation, XBOX, GameCube, Dreamcast etc. 
• All are expensive at around $30-$50 to ship.
• If you do ship, please consider some of this person's advice as well: https://tronicsfix.com/blogs/news/how-to-pack-and-ship-a-game-console