Let $X$ be a compact metric space and $\mathcal{M}(X)$ be the space of variational-bounded, signed Borel measures equipped with the Kantorovich–Rubinshtein norm, cf. [Section 8.3, 1]: $$||\mu||_0:= \sup\Bigg\{\int_X fd \mu: f \in \mathrm{Lip}_1(X), \sup_{x \in X}|f(x)|\leq 1 \Bigg\}$$ where $\mathrm{Lip}_1(X)$ deontes the set of Lipschitz functions on $X$ with Lipschitz constant smaller or equal to $1$.
Let $K\subseteq X$ be closed (with nonemtpy interior) and $M>0$ be a constant. Are the sets $$\{\mu \in \mathcal{M}(X) \mid \mu(K)< M\} \text{ and } \{\mu \in \mathcal{M}(X) \mid |\mu|(K)< M\}$$ open in $\mathcal{M}(X)$?
Remark: Bogachev shows that the topology induced by the KR-norm on the set of all nonnegative Borel-measures $\mathcal{M}_+(X)$ coincides with the topology of weak convergence. Hence, one can verify that the above sets restricted to $\mathcal{M}_+(X)$ are open in $\mathcal{M}_+(X)$.
References: [1] Bogachev, V. I., Measure theory. Vol. I and II, Berlin: Springer (ISBN 978-3-540-34513-8/hbk). xvii, 500 p./v.1; xiii, 575 p./v.2. (2007). ZBL1120.28001.