Why does it cost $4 to ship a sticker?
Because $4 is actually the bare lowest cost that USPS charges me for shipping a first class envelope with tracking.
And if I'm shipping outside WA it costs me more than $4. Also I pay my team a livable wage so it doesn't even factor in the costs for handling/labor.
The good news is that it's a flat rate so you can add unlimited amounts of stickers to your order and only $4 for shipping.
I used stamped envelopes in the past, but unfortunately they would get lost in the mail. So I ship everything with tracking and insurance now.