5 bottles of milk + IS or ARE?
Grammatically, '5 bottles...' is a plural noun, and should be followed by 'are'. But, considered a phrase, 'five bottles of milk' is singular, just like I started my comment here, '5 bottles of milk' is a plural noun.
Other similarly confusing usages are, say, a lot of examples....are/is. According to grammar rules, it should be 'is', but it is for sure plural. Still confusing is another example, 'A number of opportunities'....is/ are...
May be, others have better answers