Last year I shipped someone a used humbucker that I sold on Reverb. When printing the label, I used Reverb's recommended shipping option, which is supposed to automatically calculate the best shipping service for your item. It ended up being through USPS.
It arrived in the destination city right on time, but it was not leaving the PO, and the tracking was stuck on "could not be delivered" for days. I called, and after a customer service loop, it turned out the package was being held due to insufficient shipping fees paid - Reverb's recommended option was about $1.70 short of the actual postage needed to get it to the destination.
I asked if I could just pay the difference on the USPS website, or have the buyer pay in person, since the PO is minutes from the destination, but that was apparently against policy, and they just shipped the humbucker all the way back to me.
I had to buy an additional label (this time outside of Reverb, making sure I shelled out the extra wallet-crushing $1.70). The buyer finally received it a while later. Fortunately he was understanding. The humbucker was only worth about $60, but with all the Reverb fees and double shipping, I should have just kept it for a fridge magnet instead.