I think you need to remember that your agreement with them is a legally enforceable contract. There are various forms of contracts...express contracts, contracts implied in fact and contracts implied in law...all of which are enforceable. If there was no mention of the price reduction prior to your rendering services the client can not arbitrarily pay less than the contract price. Such is called contract breach.

What it comes down to is whether you want to continue working for less than the contract price. This is a decision you have to make. It would seem that if you waive the breach, and continue to do business further problems may result.

I know that most people on this forum are reticent to initiate suit. It is usually only a small claims action. What most of these people worry about is whether to cost of enforcement would out weigh the contract price. You need to consult your state's small claims procedures to find out if you can sue a non-resident in small claims court. If not you will need to enforce the contract in the client's state. If so, you can regain your filing fees as part of your damages. You should also research whether your travel expenses can be claimed as damages.