You can still go ahead and send to COMC if you are in Australia (unlike PSA) but if you state that you have 3K worth of cards that should incur a $300 duty based on the 10% tariff. COMC being who they are will not tell you this before you send so you may get a nasty surprise when they have your cards.
Do you just under-declare? Send 100 cards worth $3,000 and just put $200 on there?
The tariffs affect the value of the import (your cards) not the service amount. As PSA now puts an automatic $500USD value on the lowest card that means if you send them 6 cards you will have to pay a $300 duty ( 6x$500= $3000 and they have to collect 10% of that for Big D).