Catalyzes the post-translational methylation of isoprenylated C-terminal cysteine residues. Carboxyl methylation is a reversible and potentially regulated step in the post-translational modification of prenylated proteins (By similarity). [protein]-C-terminal S-[(2E,6E)-farnesyl]-L-cysteine + S-adenosyl-L-methionine = [protein]-C-terminal S-[(2E,6E)-farnesyl]-L-cysteine methyl ester + S-adenosyl-L-homocysteine Divalent metal cations. Probably Zn(2+). Belongs to the class VI-like SAM-binding methyltransferase superfamily. Isoprenylcysteine carboxyl methyltransferase family.