I dont think there's such policy.
On the other hand, $18.40 for 6 CDs with jewel cases is not so bad shipping price as for eGay (where sellers have to ask for some handling portion to cover at least partially for the fees).
The ACTUAL shipping cost on 6 CDs (approx 4oz per cd + 2oz envelope = 1lbs 10oz total weight) is $15.25 (or $11.95 if shipped in priority airmail flat-rate envelope, but not many people do that coz it takes effort to pack well).
Without jewel cases it will be marginal between $8.45 (under 12oz, minimal packaging) or $10.10 (13-16oz)