Most legal cases are settled through negotiation rather than litigation because the latter can be expensive, time-consuming and the results are not certain. If you can negotiate a settlement then you may be able to turn a would-be adversary into a customer, which will be much better for both of you in the end. Start with, " was the the infringement accidental or on purpose?" Did it occur just once or were there repeated violations, that can be proven, and when?" What are your damages, if any? Consult a good attorney so you know your rights, even if you decide not to pursue litigation.