Never ship overseas by Surface. As you have learned it takes months to arrive and most buyers want their item faster than that.
And don't think that because the deadline for Paypal (45 days) has passed that you are free. The buyer probably used a credit card to pay. PP is just the processor. Credit cards have chargeback deadlines as long as 120 days.
Your customer has 60 days to leave feedback and DSRs. If your DSRs drop below 4.3, your abiltiy to list and sell will be serverely restricted.
You can't prove that the item arrived. Refund.
And don't use Surface Shipping in future. Yes, it seems cheaper, but if the customer is willing to pay it, don't second guess her.