As I said:
‘There is’ is used when followed by a singular noun; ‘there are’ when followed by a plural noun.
There is a (or some) cake.
There are some cakes.
In a list you can use either.
“There is a cake and some cookies.”
“There are a cake and some cookies.”
Both are correct, though ‘there are’, while more logical, sounds a little strange.
In English, usages which sound correct are correct (“I could care less,”); there is no equivalent of the Acadamie Francaise to set the rules.
The basic confusion here is jumbling collective (“cake”) and individual (“cookies”) nouns together, at which point neither “there is” nor “there are” sound right.
“There are some cakes and cookies” sounds right.
So does “there is cake and some cookies” (because you’re collectivizing the whole cake+cookies phrase).
But I think most people would just say “there will be cake and cookies”, avoiding the whole problem.
As for coffee and tea, it depends whether you’re asking how much each of them costs (“How much are the coffee and [the] tea?”), or how much your total bill for both is (“How much is the coffee and tea?”).
The lesson here is not so much the grammar, but rather the fact that certain combinations can be confusing regardless of correctitude, and so should be avoided! 