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The problem is that most shavers are designed to plug into an alternating current source (the plug has a converter to go to direct current), while battery packs are always direct current.
So you need one of these two things:
A battery pack that can generate an AC output (and ideally came with a wall outlet shape)
A charge cable for your shaver that can charge off of DC and ends in a USB plug.
You are better off looking for the first one. It wastes power, but I just don't see a company making a USB DC to DC cord.
Edit: Here is an example found on Amazon
Edit 2: More search results
AC Outlet Portable Charger... [link]
How about this one ?