I have been offering $2.00 shipping for years on lower end cards and free for higher end and have not been dinged at all. Sorry to hear the issues with your buyers. Can you link us your profile so we can take a look?
Collecting Rickey Henderson(especially Graded 9+ TCMA '79 Ogden A's Minor League Card)
Jerome Bettis, Rod Woodson, Greg Lloyd, Kevin Greene, Antonio Brown, Le'Veon Bell, Markus Wheaton