“Now, there’s two reasons”. Is that grammatically correct, and if not, what is?

In the TV show “Friends”, Chandler says, “Now, there’s two reasons”. Is that grammatically correct?

This question, found on Quora, asks about the following structure.

[There] + [be]

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Formal language

In formal language, [ … there are two reasons … ] is OK, although [ … two reasons exist … ] is more precise and probably better in most formal contexts.

[ … there is two reasons … ] or [ … there’s two reasons … ] would be bad style.

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Informal language

[ … there’s two reasons … ] and [ … there are two reasons … ] are both fine. Occasionally the second is written [ … there’re two reasons … ] if quoting speech when [are] is pronounced as a “schwa”.

[ … There is two reasons … ] (without stressed [is]) would be odd, as the full [is] is formal, but the grammar is not canonical.

We could imagine the following conversation: “There’s only one reason, there isn’t two reasons.” – “But there is two reasons! ….. ” – The second sentence sounds a little odd because it’s more obvious that the grammar is “wrong”; normally we would reply “But there are two reasons! ….. ”