Once your stake is complete you can start spending and you will be automatically credited back on all of your purchases.  If you stake 145,000 TERN (at a rate of $0.008) and spend $20,000 on BlockCard, then your stake has paid for itself.   

$20,000 * 6% = $1,200

145,000 TERN * $0.008 = $1,200

You will also earn TERN rewards on your rewards.  This means you can take your $1,200 in TERN and spend it on the card, yielding another $72.  Anyone can create an account, but currently, only US citizens can get the BlockCard.  A banner on their website also said 31 European countries would be open soon for the card.