Hello, my name is Jim and I am a qualified lawyer happy to help you today.
The Consumer Rights Act 2015 applies here - so if you purchase online then you have an automatic 14 day cooling-off period. After that, if the retailer gives you a returns option then you can ask for an exchange as per their terms and conditions. If they do not then you can allege breach of contract. Are they saying you need to post it back?.
I would send the retailer a letter before action to demand payment within 14 days and say that if they do not pay you, you will issue county court proceedings against them. See attached template as an example of what to say.
Assuming they refuse to pay, you would then need to issue a claim using the simple procedure in Scotland here:
For a claim up to £300 it costs £19 to start the claim. For claims over £300 (up to £5,000) the fee is £104. If you are in receipt of certain benefits you are exempt from paying the above fee. The fee exemption can be applied for at the above website.
Once you start the claim the Sheriff will register the claim and a timetable will be given to you to detail how the claim will proceed. For this size of claim you can easily do it yourself without a lawyer (or come back to this site for further guidance).
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.