IF you formed a contract / order to supply them the goods requested and you didn’t separate the design part (charges or rights reserved) then the purchase of the design was implied as they could not receive the goods without also getting the design for their sole use.
Yes, you may be the designer and hold the copyright, but during the sale of goods it could be implied that you sold the design also. The user holds significant rights in the real world as you didn’t restrict the use in the original terms of supply.
Sucks I know, but how many times have we all been asked and replicated a company logo whether national brand or a local merchant and done it. I know I do – and it’s done on here every hour of every day when people ask and receive reworked logos / brands for non-sanctioned use without any thought for the original designer or manufacturer.
Move on or ask to re-quote – don’t waste your time or effort unless you see £££ rewards and can support a potentially VERY costly lawsuit.
Dave