.1 BTC is definitely very low for the instant buy.  In my experience, there was a 30 day hold period(Buy and hold on to BTC) along with registering a credit card for the instant buy.  After that, you can buy up to 10 BTC instantly a week. How it works is if you buy x amount(up to 10), the amount of instant buy starts replenishing at a rate of (10BTC/7Days).