I recently returned a product I bought from eBay. PayPal returned the money directly back to my credit card. Then they returned the postage I had to pay, but it went into my PayPal account. In order to get it back they want me to link a bank account. It sounds dodgy because they have my card details, so why not just put it back there?
It sounds like the postage amount was paid to you rather than returned. If it had been returned and the payment originated on the card, they would have to return it to the card. If it was processed as a payment, it looks like someone is giving you money. PayPal can't credit it to the card, as the sender could request a refund. If PayPal put the money on the card against a previous payment, then they wouldn't be able to refund.
If they add money to your bank account, then they can withdraw it if a refund is required.
One reason that you might get a payment is if you were being reimbursed for spending money outside of PayPal. If the amount is more than you originally paid, they can't put it on your card. They can only refund to the card. They can't deposit to it.
If you don't want to give them your bank account information, you can just wait until the next time you use PayPal and use your balance to pay. Then you can bill the remainder to your credit card.
If you don't normally use PayPal and just want your money back, you can process a chargeback through your credit card. Note that this would probably annoy PayPal, as it costs them aggravation and potentially money. To do this, you must have paid the postage with your credit card originally. If you spent money outside PayPal and were reimbursed through PayPal, then there's nothing to chargeback. In that circumstance, you'd have to accept one of their options: pay with balance or deposit to bank account.