Got a better solution than this.
You can buy some non-returnable items worth more than the amazon gift voucher.
Now place return request through customer service. Sometimes there may be a little back and forth. But normally if you flag it as "defective item, doesn't work" they will mostly provide refund.
Now mention them simply that you want the refund in amazon pay balance rather than gift card.
It worked for me. I accidentally discovered this while returning an unwanted item bought via amazon gift voucher. Also, as a bonus, they didn't even picked up the item and it was worth 3300/- rupees.
I successfully converted my 3300/- amazon shopping voucher to amazon pay balance.