I performed a service with no contract and no receipts of payment and now the costumer is saying I owe them money for more services.

If you had an agreement to do specified work for specified compensation, you had a contract.  This does  not sound like a type of contract which is required to be in writing to be enforceable, like a contract to sell real property or a contract which can't be performed withon a year. As it was not written down, the question is what the terms of that contract were - were thay as you say (i.e. you performed all the work required) or as the customer says (i.e. you agreed to perform more work, or better work, for the money you were paid).  If you can't reach an agreement with the customer, a judge or jury would have to decide. Complicating this is the fact that you had no license.  Depending on what kind of services you performed, you may have been legally required to be licensed, and your contract may be unenforceable without one.  This could mean, again depending on what type of work you performed and local MT law (with which I am not familiar) that you would either have to refund all the money, or that you would only be entitled to keep the fair market value of the services you performed, even if that is less than the amount contracted for.