Please excuse me answering in English, but my German is not that good.
Congratulations on this cover. It is actually correctly franked (see here: http://www.sijtzereurich.com/postal_rates_Latvia3.html)
15 rubles (100x 15 kopecks) was actually the rate for an international airmail letter of the lowest weight class. Pastende is a very small place in the countryside in western Latvia.