Well, that's cheap enough, but it's blindingly obvious that that mob have no clues about calculating acid concentrations.
Commercial 'syrupy' Orthophosphoric Acid is 85% H
3PO
4, and diluting that with 4% demin water sure doesn't get you to 96%!
Their packaging looks a bit suss for that product too, unless it has a back label with the hazard diamonds for Dangerous Goods Class 8, and other safety info on it.
The 'thickened acid' rust removers are sold at paint and hardware shops, as either 'rust removers' or 'rust converters'.
The other commercial 'rust converter' chemistry in use is Tannic Acid based, but this type is less common than phosphoric acid, and much more expensive these days.
