i will throw in an often overlook aspect. Buyers like to kid themselves (and their partners) they got things cheaper

eg

$29+ $8= Look i go this for less than $30

vs

$37= not sure i want to pay over $30 for it / you paid HOW MUCH for that?

ie everyone likes to kid themselve how savy a buyer they are, and often delude themselves over the real cost. Add to which there is a buyer perception that free post prices are loaded.

Ebay does not boost freepost items in searches as much as often thought, if at all.

I choose a subsidised option, ie a flat rate which is less then the minimum regardless of what it turns out to be ie $5 whether it end up in a 500gm or 3kg satchel. That way, no need to estimate it (and many of my items are borderline). No disputes if i get it wrong. It gives a facility to discount postal rates for combined if necc, in many cases no need to as 2x$5 is still under price of a 3kg satchel even 3x isn't loaded especially if using signature.

A lot depends if it is a common item with definate market value or not, if they are individual items with suck it and see values then perception plays a bigger part than actual total $ price point