I only sell at the pack/case level on Amazon. Do I need to provide all levels of unit barcodes/GTINs?

No, the single unit of use GTIN will be sufficient. If the single item itself is in a pack or case, as well as any other packs/case configurations will also be sold or used by Amazon in any way.

Vendors should share all GTINs from a same underlying item package hierarchy (e.g. pallet, master pack, inner pack, each). This will be mapped to Amazons existing catalog.