advice please, FBM and I’m new. UK

I’ll paste my interaction with the customer:

“Customer:

Hello, I’m looking to return this item, I’ve used it and believe its not good enough/ not compatible with my device, so i would like to return this item, is this a possibility.

Me:

Hello and good morning, yes, of course that’s possible my friend. please create a returns request via your order and we can go from there :). once the cells are in my possession, a refund will be dispersed the same day. (I do work a full time job on top of Amazon/eBay, so please bare with me in terms of timeframe) thank you

respectfully, me.

Customer:

Hello,

I’ve created the return request, is there anyway i could return this item, without paying to post at the post office. Kind regards”

How do I go about this?