I don't think you can claim it back if you aren't present in that country  when you make a purchase and are about to depart from it (and haven't lived there for more than 6 mths),  or if the seller hasn't deducted it from the sale total before payment.

When I was overseas (UK & EU), had VAT receipts for purchases, the retailers took a % (commission) of the amount refundable, the receipts had to be presented to customs officer before departure at the airport, who stamped each one individually. Then they could be put with a claim form in the post and wait for the refund to a c/c.