The Global Shipping Program is run by Pitney Bowes and not by eBay.
Where you went wrong (and it is not clearly spelled out) was in not mentioning the GSP in the dispute.
That should have moved the dispute to a GSP specialist.
PB/GSP actually has a good reputation for settling disputes.
- Seller doesn't respond at all
While that is rude, the seller's responsibility for delivery ends at the GSP plant gate in Kentucky.
And the seller did not get the shoes back either.
Now the 30 days from last estimated date for delivery has passed and eBay will not help.
Go instead to Paypal.
You have 180 days from PAYMENT to open a PP dispute. The PP Resolution Centre is at the top of the page under Tools.
Same basic system, but this time do state that this was a GSP transaction.
If that doesn't work out, you are also covered by the chargeback provisions of the credit card you back your PP account with. The customer service number is on the back of most credit cards. Card policies vary, but have dates, currency and payment information handy to help the clerk.
Also-- in that order.
If you start a cc chargeback, PP will not entertain a dispute. If you start a PP dispute, eBay will not allow an eBay dispute . The reason is to prevent double-dipping.