As someone who hand makes the majority of my own items, I think you are well underpricing your cards.
I looked at a few cards and they all had FREE postage, so you might want to double check this.
My suggestion is that you never offer a buyer the choice of free or tracked postage.
Either add the tracked postage cost in one of 2 ways -
1) Build the price into your item (yes, you will need to charge more)
2) Increase the price of your cards (they are too low, as I mention) and also add tracked domestic letter postage.
Some of your cards are the DL size which is $3.05 (charge $3.50 to account for fees)
Some of the cards are the A5 size, which is $5.10 (charge $5.60 to account for fees)
Personally I would be pricing cards at least $8-10 plus postage as you're probably not even covering your material costs right now. Yes it seems expensive, but there's 2 ways to look at it - at your current price point you are literally giving them away, plus you're having to refund. At a higher price point, you may have slower sales, but if people want handmade, they will pay a premium. And you're protecting yourself in case the item doesn't "arrive".
Also, move out your handling time - mine is currently 10 days, even though I usually post within 48 hours.
