We see what Chambers did here. Jacob is a tourist in a far away place—Amsterdam—and what do you send when you're a tourist? Postcards. We're thinking the title is a play on the fact that Jacob is far from home throughout the book—both literally and metaphorically. So too, of course, is Geertrui in her own way.
On a number of levels, both Jacob and Geertrui go places they've never been before—a foreign country, yes (for Jacob, anyway), but also in terms of sex, death, and other major life stuff—and in the process, learn a whole heckofa lot about themselves. So while Jacob literally goes to Amsterdam in this book, that's really just the beginning of the journeys we watch unfold.