Learn German: Darüber hinaus bekommen Sie heute das Zubehör gratis dazu. - Beyond that, you get the accessories free of charge with it today.

Because the sentence starts with the adverbial connector Darüber hinaus ("Furthermore" or "Beyond that"). In German main clauses, the conjugated verb must always take the second position. Since Darüber hinaus occupies position one, the verb bekommen takes position two, pushing the subject Sie to position three.

The English translation says "accessories" (plural), but the German uses the singular das Zubehör. Is there a plural form?

In German, das Zubehör is a collective noun, meaning it describes a group of items as a single unit (very similar to how we use "luggage" or "equipment" in English). It is almost always used in the singular, even when you are talking about multiple accessory pieces.

Both Darüber hinaus and dazu express addition. Why are they both used in this sentence?

They serve completely different roles here. Darüber hinaus is a transition phrase connecting this whole sentence to a previous one ("Furthermore..."). Meanwhile, dazu means "with it" or "along with it" in the context of the deal, showing that the accessories are included with the main item the person is buying.

Could I say Darüber hinaus bekommen Sie das Zubehör heute gratis dazu instead?