Okay gotcha.
In that case, I would make it clear in every listing: If expedition is required, please notify me in the notes to seller or send a convo in advance of your purchase so that a shipping upgrade can be made in the listing. If purchase is made first, order will not be shipped until Paypal invoice for the shipping upgrade has been paid.
You could include an expedited shipping price range estimate if you find a lot of expedition requests are coming your way.
Kat is right in that you can't list a shipping upgrade as a stand-alone listing in your shop, so sending your customers a Paypal payment request is the best thing. Don't ship anything before you get payment for the shipping upgrade, and make sure shoppers know that. Be open in your communications- if a customer requests expedition, contact them via convo and email stating that a Paypal payment request will be sent to the contact email listed in their etsy account, and that their order will be shipped once the payment is confirmed. That way they'll know to babysit their inbox and pay as quickly as they can.