Without meaning to put a damper on your technical work, you should keep in mind a few things:

-- systems that work with money are attacked hard and often, by intelligent skilled people

-- in fact some of the people who attack your system are likely to be both more skilled and more intelligent than you are

-- systems that work with money that fail, fail spectacularly ("What do you mean someone withdrew $8 million last night?")

-- banking websites, Paypal, etc. are all like icebergs - you don't see 9/10ths of the things they've done to prevent spectacular failure

-- spectacular failure is your destiny if you don't work very hard to prevent it

-- spectacular failure may be your destiny even if you do work very hard to prevent it

You should plan accordingly.