To see your recharge settings, access the 'billing' option from the menu at the top right of the dashboard:
Then, scroll down to the recharge option:
This is the amount your card will be charged whenever your thanks.io balance drops to zero.
Anything your purchase via thanks.io will leverage your available balance, and then re-charge your account if the amount drops.
Example 1: Bill has a balance of $52.75 and a recharge amount of $100. He sends 108 $0.49 4x6 postcards. He is 17 cents short, so his card is charged $100.17, and leaves him with a credit balance of $100.
Example 2: Stacy has $5 in credits and a recharge of $10. She purchases a mailing list and the list is $10.01. The list purchase uses $5 in credits, and then charges $5.01. Since this leaves a balance of zero, it recharges $10 leaving her with a $10 credit balance.
Hint: Your transaction history at the bottom of the billing page will have all charges listed for review.