In terms of getting a refund for the shipping costs, you would have to contact the shipping carrier themselves as that's where your payment goes to. If I were in your situation, because of the time limit that eBay has for shipping, I would pay the shipping cost difference. Otherwise, unless if the buyer is compliant, you could receive feedback that could set your sales back. If the buyer is compliant, let them know, in detail, how long it could take for you to be be able to ship the item (example, "It'll take 30 business days to ship it"). Don't ever put ambiguous information, such as "It'll take a week or two to ship it." Also, let them know what's going.
If you're unable to pay the difference, then I would let the buyer know what's going on and how long it will be to send the refund. Again, don't ever put ambiguous information, such as "It'll take a week or two to refund it," only detailed information, such as "It'll take 30 business days to issue a refund." If your buyer isn't compliant, then you're out of luck because the buyer could submit a dispute either through eBay or PayPal.
Good luck, @Hay15! Let me know how it goes!
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