Shipping when buyer buys multiple of the same item

[whiteexpy]

I have an item listed and it fits in the USPS Priority Mail Medium Flat Rate Box, which is a fix shipping cost.  The customer purchases 3 and still charges the cost of shipping for 1 item.  The box cannot hold more than 1 item.  How do I fix this?

[rfmtm]

In your Shipping Preferences, if you have Combined Payments turned "on" , then edit the Flat Rate and/or Calculated shipping rules applied to the listing to "no discount".

https://www.ebay.com/ship/prf

IF you have Combined Payments turned "off" then on the listing make sure both the first field for shipping cost of the first item and the second field for shipping cost for each additional item are filled in.