VAT refund after leaving Germany?

The process to get a refund normally works as follows:

• The store gives you a form for your purchases
• At the airport, you get the form stamped by customs
• You then give the stamped form to someone who will process the refund. This can be done either by visiting a tax refund office at the airport (note that this is different from the customs office) or by mailing the stamped form back.

If the form was stamped, just return it by mail, there should be instructions on the form for that.

If you couldn't get the form stamped, https://www.germany.info/us-en/service/09-Taxes/vat-refund/906296 tells us:

Certification by German diplomatic or consular mission (exception)

In very rare exceptional cases a certification by a German diplomatic or consular mission is possible. A plausible explanation has to be given in writing why it was not possible to receive the export stamp at the port of departure from Germany.

The same procedure as at the customs service applies:

• original receipt
• export papers or Tax Free Shopping Check
• purchased goods (unused/unworn in its original packaging and with price tag)
• and your passport together with proof of residency have to be presented to a consular officer for inspection.

Please note that the certification can only be issued for goods purchased in Germany (for goods purchased in other EU member states please consult the respective Embassy/ Consulate).

Make sure to present the goods to the correct German mission. There are nine German missions in the United States. Each of them covers a specific geographic area. To find out which one can assist you, please use our Consulate Finder.

Please note, that one of the criteria to receive the tax exemption is that the article(s) must be exported before the end of the third calendar month following the month of the purchase.

If the purchased goods are presented to the competent German foreign mission later than that, the time of export has to be substantiated through appropriate documentation (e.g. airline ticket, entry stamp in your passport).

There is a fee of 34.07 Euro for the seal on each individual export paper. The fee is payable either in cash (equivalent in US-$ according to the daily exchange rate) or by internationally accepted credit card (card will be charged in Euro, additional charges by the credit card company may apply).

(emphasis mine)

The page is geared towards visitors from the US, but I suppose the same principles apply elsewhere.

Yes, mail is the only way possible to do so as you need to send the original, custom-stamped, invoice back to Germany in order to be issued your check (beware, it will be in euros)

Official info :

https://www.germany.info/us-en/service/09-Taxes/vat-refund/906296

(emphasis mine)

After having returned, the stamped invoice should be sent back to the store in Germany where the merchandise was purchased. For the Tax Free Shopping Check you may be able to obtain a cash refund if they have an office at the airport. Otherwise the check should be sent to the address on the back of the Tax Free Shopping Check where you will find detailed instructions for the refund.