Although the end answer may not be any different, it would help if you would be more specific.
Please tell us the name of the shop
what items you bought
what items you returned
what is the value of the alleged overpayment to you and to which returned items does this apply?
Don't touch them owe me £500 since January 2019 make excuse after excuse. Seem they always have software problems sending money out. Keep saying they will call back or email nothing been chasing it now for 6 mths the phone staff always have the same banter we will chase it up and get back to you then nothing!