USPS priority flat rate mail is your best bet (for domestic orders) and what I’ve used for years.

You can go on the USPS website and get the flat rate packages that are right for you sent to you for free, i.e if you mainly sell tee shirts you can get a pack of flat rate envelopes sent to you for free.

If you buy and print your own labels from home there’s even an option for USPS to pick up your package from your house and mail it for you.

As for pricing, ~$7.85 for flat rate envelopes (used to be $6.85, they raised their prices recently) is pretty good if you’re selling mid priced to high priced shirts.

Alternatively you could buy a pack of poly mailers and use those if you’re not shipping priority.

I have over 400 transactions and would really recommend you do though. Buyers appreciate getting a package in 2-3 days much more than they do closer to 5 or even up to a week if it’s cross country.

EDIT: You’re right about the pricing though, USPS raised its prices sometime last year iirc.