Sulfate dianion is a very common counterion. Here we have sulfur in a #VI+# oxidation state. For #"thiosulfate ion"#, ONE of the oxygens has been replaced by a sulfur. And in the replacement, sulfur has assumed PRECISELY the same oxidation state (#-II#) of the oxygen it replaces (and such a formalism is reasonable, because sulfur and oxygen are in the same group!). The average oxidation of both sulfurs is still #+II#, but we can individually assign separate oxidation states to the central and terminal sulfurs.
Both sulfate and thiosulfate ions have resonance structures, typically #""^(-)O-S(=O)_2-O^-#, and #""^(-)O-S(=O)_2-S^-#. Of course, the negative charges are delocalized around the chalcogen atoms.
