If your needs are simple, you have a lot of options. If you're looking for a long term partner that has a wide toolkit - Stripe. For example, you can offer indirect billing (i..e the lawyers can bill their clients via your platform - you earn a commission); or you have other business models, receive from a customer, pay out a commission - Stripe platform. If you are looking for the least expensive option and don't envision your needs evolving - Chargebee.
