Yes, I was thinking the same, box is singular, but even in plural we would say in english 3 boxes of cereal. I also don't understand why it is de and not des, if cereal is to be plural, or is boîte de a fixed phrase?
If you have "a box of (something)", it is just going to be "une boîte de (qqch)". EN "cereals" here is incorrect--the result of an overtranslation from the French. English speakers do not a "a box of cereals".