USPS has your lowest min pkg cost. Fedex and UPS are around $7 I think for most accounts? Maybe lower if you have a monster account but even then it's typically with a USPS partnership that they get it done.
1st class in a polybag is our go to for singles. Sans food sealer, haha. I kind of like that idea.... but not the part of the story where somebody gets a vacuum squished shirt.