This paper deals with the existence of optimal transport maps for some optimal
transport problems with a convex but non-strictly convex cost. We give a decomposition strategy
to address this issue. As a consequence of our procedure, we have to treat some transport problems,
of independent interest, with a convex constraint on the displacement. To illustrate possible results
obtained through this general approach, we prove existence of optimal transport maps in the case
where the source measure is absolutely continuous with respect to the Lebesgue measure and the
transportation cost is of the form $h║x-y║$, with h strictly convex increasing and $║.║$ an arbitrary
norm in $R^2$.