I generally go USPS Ground Advantage, finding it far cheaper than flat rate boxes, which must go Priority Mail.
Get an online USPS account, an accurate scale, and use packing tape to affix labels to good boxes.
"Buyers" may balk at $15 Priority Mail shipping over an $8 charge, otherwise.
Easy to collect boxes from your own Amazon deliveries. Or ask neighbors to save for you.
I purchased many boxes of varying sizes for roughly $1 per. Sometimes you can get freebies from nearby businesses.