Pirate Ship is cheapest, they have discounted US Postal Service and UPS rates. Yes, there are Bitcoin-accepting services online that you can buy postage from, but they charge more than the post office by a couple dollars at the least. I've used them. They all use USPS, so why not pay less? I buy Bitrefill Visa ecards with sats, and use those to pay for postage through Pirate Ship. Hope this helps 👍