The relative clause here (that is on the top shelf), gives us extra information about the trophy. We already know which trophy is meant so we use comma. It's called non-defining relative clause. Suppose the listener already knows about the trophy. If the listener doesn't know so there shouldn't be commas, the relative clause is a defining one and we need that information and it's not extra information that we already know.
There are a few slips there.
If the relative clause is
that is on the top shelf, then it can be only a defining relative clause, telling us which trophy is on the top shelf. There are no commas,
If the clause had been giving us more information about the trophy, then it would have been a non-defining clause, within commas. However, we normally use 'which' rather than 'that' in non-defining relative clauses.
