eBay's policy states:

"A shipping and handling fee can cover the seller's reasonable costs for mailing, packaging and handling the item.

Violations of this policy may result in a range of actions, including:

• Listing cancellation
  
• Limits on account privileges
  
• Account suspension
  
• Forfeit of eBay fees on cancelled listings
  
• Loss of PowerSeller status"
  

http://pages.ebay.com/help/policies/listing-shipping.html

I don't see how $20 shipping for four slot cards could in any way be construed as "reasonable."