third-party chat service providers

I need to develop a one-to-one chat feature in a php web application. Right now I am evaluating what is the most cost-effective way to go. My major concern for building a DB backend on my own is that, as time goes by, DB will be filled up with more chat history and eventually cause more money (since we need to pay more cloud space).

I want to know if there is any alternative third-party chat service that I can integrate. I am expecting this service to save my chat history on its own backend, and I have to pay a relatively small upfront cost.