You just have to put it out there. 'Free shipping' and build the cost into the sale price, or "I will ship on your dime, I use "fill in the blank"".
Sooo many variables. USPS can vary simply because the counter person measures differently. For smaller items they have "if it fits, it ships" flat rate priority boxes but you need to compare, because parcel post for the same size/weight box is often still delivered in 3 days and can be half the price.
I use a blank box and buy a roll of Priority tape. I used to invert the (free) priority boxes until they started printing on the inside of the box too.
My experience is UPS is the most expensive, but offer the best delivery times to a greater range. FedEx is my "go to" because they're close, open until 8pm and significantly
less expensive than USPS/UPS for the same package.
I've used Greyhound for bulky, odd sized/shaped items (bumpers, door cards, hitches). Super reasonable, but there's no telling exactly when it will arrive at its destination and there's little to no tracking.
Once upon a time when eBay was a real auction site (I signed up in 2001), I shipped hundreds of item per year. Times have changed.