OK, what's the protocol?

I personally will give my best estimate, and if it's less refund the difference, if it's more within reason, I pay it.

I would consider the actual cost of driving to send the package, and such.
If the charge for shipping was over more than a few dollars, I would offer the buyer a refund.
If it was less than a few dollars, I would count it as gas money.
In some cases folks charge a bundle for "time spent" and expect to make money on the whole thing...
Other kindlier folks just want the buyer to enjoy the product as much as they themselves did.
A Gentleman would ofer the refund of the extra cash for shipping. And a Gentleman or Lady would decline the offer.
Also if the shiping chare was too low, and you had to pay out of pocket.
A Gentleman would politly mention the extra cost, but tell the buyer not to worry. And a Gentleman or Lady buyer would insist on paying the extra charge, and the seller should gracefully accept.

I refund the difference.

Let your conscience be your guide.

Gas money today is probably more than a few dollars!