It depends on the terms of the embargo and how it is enforced. Also important in this much reduced EU is how borders are managed. If there is already a system of customs checks and tariffs for any goods leaving Germany then this makes it relatively easy to prevent any legal exports to a specific country.
Of course one problem is that Germany and Spain don't have a common border so this is potentially easy to get around by using another country such as France as an intermediary.
In the case of an international company like amazon it would be easy enough just to route goods from a warehouse in another European country, in fact I doubt that there is that much direct retail trade (in the sense of ordering a package in Spain and having it shipped from Germany) between Germany and Spain in any case. Obviously the details of this will depend on the exact legal framework of the embargo.
It might effect small independent retailer more, especially if they sell/manufacture a unique product themselves, say hand-made musical instruments for example. But again this sort of thing is a pretty niche market which may be not worth enforcing.
Looking in more detail at german exports we see that the vast majority of goods manufactured in Germany are vehicles and industrial parts and very few things which are likely to be ordered directly by a consumer. Even cars generally go through a long and complex retail chain of importers, agents and dealers.
With this in mind it is also quite likely that any embargo simply wouldn't bother to include retail transactions in its scope as restrictions on bulk exports and financial products are much more economically significant and easy to enforce.
There is also the consideration that an embargo usually requires a large block of countries acting together against a specific target. Trade is, by definition, a mutually beneficial arrangement so two individual countries refusing to trade with each other is not really much more than a gesture unless one controls a commodity that the other needs and can't easily obtain anywhere else.