You will have to do some research. We went through the same issue on how to send our cigarette cards.

Do you have enough padding in your cards to perhaps absorb some postage?

Must they be Registered?

Maybe instead of automatically sending Registered, offer as an option to the buyer with the knowledge that will be dearer. Then the buyer makes the commitment to pay the extra, ie taken out of your hands.

If you offer lower postage, (unless they choose the dearer option) you will probably sell more, and this could help offset some of your postage expenses. 

What are other sellers doing? Especially long time sellers, they have usually got it sussed.

How do you pack? Could you use different packaging?

There is a CD box available BX7 costs $1.55 from PO, (suits CDs and DVDs),  

another option BX9 Mailer, (Suits CDs and Photos) $1.25 from AP, but could buy from online sellers a bit cheaper, perhaps these could be alternatives? 

And there is a hard white envelope from online sellers that comes in DL and also C5 size, (and possibly larger)  We use them for our cigarette cards, gives the strength of cardboard and as long as doesn't go over 20mm thickness will go as letter rate. Not tracked but gives some protection.

I often stroll along the PO shelves looking at what they have for packaging and have found a couple of items that we use, or bought similar online.

Will take some time to work out what is the best option, probably will depend on value, location and quantities sold.

Good luck.