why would they be interested in the fact that you are owwed some money by the shop but didnt wait to collect it.
This is the nub of it, you havent done anything wrong and leaving without sorting out the refund has only put you out of pocket
your dad can represent yu but that normally rules him out as a witness.
As for suggesting what to write- we dont have a clue as to what has gone on so not in a position to advise othe than the most general of things.
Send everything you have and intend to use as you wont be abvle to add anything not covered by your statements later. If in dount leave it in.
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!