If you buy first class postage online, it will probably give you a delivery confirmation number, which is sort of like a tracking number, only not as reliable. For whatever crazy reason, it's not available if you buy in person from the local post office.
If you want a true tracking number, you have to buy priority international shipping, which is super expensive, like $50. It's expensive because it's also quick delivery. And the private carriers - FedEx, UPS, DHL - are all the same. Tracking is only available with priority shipping. They're all about $50 because they're all quick delivery. So basically, if you want fairly reliable shipping at a reasonable cost, online first class international postage is your best bet. It should give you the delivery confirmation number automatically; you shouldn't have to request it specifically. It's not available to all countries, but I would bet Germany would probably be OK. Delivery should take around 2 weeks.