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- Jerome Bettis, Rod Woodson, Antonio Brown, Le'Veon Bell, Jarvis Jones
Non Steeler Side Projects- Eddie Lacy, Marcus Lattimore, Michael Carter Williams