Received a legal notice from a client

Hey, I have received a legal notice from a client stating the charges of 405 & 420 I would like to brief you a bit. My partner & i met this client few months back where we agreed on a certain deal where in the client gives us raw materials and we give them finished goods we gave the client the cheapest rates possible in order to maintain long term relationship where in we also stated that if anything goes wrong we’ll do the corrections and won’t charge for the corrections part anyhow ‘no verbal or written contract’ was made which would state that we would pay them for any damages in any form, the work was a bit delayed from our end as the raw materials we received were new for us anyhow we submitted them the first batch which was 50% of it, they returned it all stating being unsatisfied so we took it back started with the corrections they mailed us again asking for all the goods back and that they would get it done from somewhere else we spoke to them and came to a conclusion that they would give us a last chance Anyhow we got another email the very next week again asking for everything back and this time without hesitation we gave everything back Let me add to this the transportation was to be on their hands but they didn’t we paid for each times transport even this last one, a week later we received a call stating that they need the money for their goods along with the advances they paid us We had so far spent way more than what we had asked for anyway they threatened us with a legal notice , my partner visited them and apologised everything was said and cleared until yesterday when we received a legal notice stating that there has been a fraud & they want to recover the cost of goods and and the advances We have no written contracts stating that we would pay them any sum in what so ever the case would be nor do we have any verbal contract Please help what can be done and is the matter serious?