Can't seem to find an answer to this question. I'm transiting through MUC (final EU gateway) and will get the necessary customs stamps on the VAT refund forms for goods purchased in Spain and France.

Two of the merchants gave me postage-paid envelopes (one in Spain (marked "No necesita sello. A franquear en destino"), the other in France (marked with a huge "T"). Are these valid if the envelopes are mailed from Germany or do I need to add German postage? (It's not a question of cost--I just have a fairly tight connection!)

A third merchant gave me a self-addressed envelope with no prepaid postage. Anybody know where I can buy stamps at MUC? Searching the MUC website only shows a Deutsche Post outlet in the MUC Airport Center, wherever that is.

Alternatively, is there any reason I can't simply mail the stamped forms from the US on my return?