I wouldn't worry about it too much man, as long as you got the card and it was in good condition great, Ive brought alot of things beside card where the postage was like $8 but it cost the seller $2 or less to send it, you also got to consider things like the cost of the envelope, if he/she had to go to catch a train/bus to post the goods etc, sometimes Australian post are too lazy to mark the stamps and I recycle them and use them again.
On other note, I actually sent a item today oversea, I only charge $25 Aussie including insurance, worked out to be $15 extra so the buyer got a good deal with postage but he did pay alot for the goods so I didn't mind.
Sometimes companies and individuals send items from oversea to me they charge me $50 Aussie, it should be much cheaper but they assit me that its the cheapest, sometimes you can't teach a old dog new tricks, if they are going to sent it expensive or express they will but there are reasonable sellers who actually say there is a cheaper postage option or in some cases give you free postage, when people send it express its usually well protected, it all works out at the end.
Oh 1 more thing I got charge $7.50 which the seller said it was insured and sent express but I got it today and it said first class internationally and the postage said $3, if the item got lost it would of been the sellers fault but lucky it came in one peace, wasn't protected well but it was in a old magnetic holder, its better than a top loader, the card was worth about $50 US, its all good, left the seller with positive feed back and said postage was good, because it came to me and was in good nic.