You can mark dispatched by opening the original prepaid label (Get prepaid label > print label).    It will remain unused and assuming the buyer paid for it the cost will be automatically refunded to them in 30 days. 

You will get the money but as an untracked item it will be held for 14 days.   It's possible that it may be held for 30 days because you're a new seller.   That is a separate issue.    Either way you will get in the end.