I say go ahead and out them after 48hrs if no resolution... the number is the number.
I've bought/sold many parts here, rennlist, etc. and always err on side being fair.
Trust must exist or the whole system breaks down to nobody's benefit. That said, freight / shipping costs are super easy to calculate using online tools anymore (ups, fedex, usps, LTL, etc.) so I'd just protect yourself up front and say part price is X plus shipping. Then just send them final amount which includes putting it on their doorstep and don't ship until you have all the funds in hand. Or as w/ most e-comm you bake freight it into the price and don't have to worry about those who balk or try and flake out on paying the freight after.
Good luck
