You can feel for small shops in this situation, maybe they only keep one in stock and have ordered a replacement now. You come along and want to return a perfectly good item and they now have more money tied up in stock than before you came in.
I would think through the possibility of offering them a restocking fee if they say no, so offer say 10% less refund than the cost of the item to allow them to cover their extra costs on your lost sale.
If its a pushchair and its a small business then is it a specialist baby shop? If so maybe take other goods to same value or a credit note for future use (suggest you use soon though, you never know how long any business will last in current circumstances). I am assuming you are getting a pushchair due to having young kids btw