We received a bill for underpayment penalty and are responding to request a waiver by sending a letter as well as the first page of form 2210, where do I send this to?

The IRS notice/letter should have instructions that you can follow.  If the notice doesn't say anything, you may want to call the IRS phone number on the notice to get specific guidance as it relates to your account.

Please also see this reference from the IRS that gives specific instructions on reply to a letter or notice.