Sustaining innovation is when a company keeps improving a product and raising the prices. Stripe is more of a sustaining innovation than a disruptive one. Typically incumbents are the ones to improve their products while raising the price, but it is also possible for a new entrant to do the same. Providing a much better user experience could potentially lead to Stripe dominating the payments market but it is really just a less hassle version of what is in place today. Payments are a huge hassle to deal with as a merchant/developer so this is still an innovation but just not a disruptive one.
Dwolla is a better example of disruptive innovation in that they are circumventing the whole credit card system which could potentially save merchants and indirectly, consumers, large amounts of money.