I’m with OP, and I’m a retailer. Doesn’t matter for what reason it’s being returned from what they’ve put seems like a CRC cock up to me. If they offer returns, and a collection service make it a decent one. Buyers remorse is allowed…
If id have been asked to delay an item, id have delayed it not made it difficult. If I’d agreed to a return collection and our courier had cocked about I’d not expect a customer to suffer because of it. Saying no to returns or paid postage is one thing if that’s how you want to play it, but if you agree to something stick to it. So no, not unreasonable after 2 attempts, though I would imagine the OP hasn’t just taken 2 days off self employed work just to do this.
A return like that is going to cost time and money, nothing you can do about it so you may as well make the best of a bad job and keep the customer happy with the level of service and make it back next time.