In the crowded field of online payments, venture capitalists are betting Stripe Inc. is a standout worth more than a billion dollars.The payments startup has raised about $80 million in new funding this week from venture-capital investors including Khosla Ventures, Sequoia Capital, and Founders Fund, said brothers John and Patrick Collison, the company’s co-founders, in an interview. Stripe, at just over four years old, is now valued at $1.75 billion.