Here's an excerpt from our Flood Insurance training manual on this topic:
2. Force Placed Overlapping Policy Refunds [§339.7(b)]:
a. Confirmation of Insurance:
i. Declaration Page Confirmation:
A bank …shall accept from the borrower an insurance policy declarations page that includes the existing flood insurance policy number and the identity of, and contact information for, the insurance company or agent. [§339.7(b)(2)]
ii. Other Acceptable Confirmations:
…a declarations page is the one option that a lender must accept, there are circumstances in which a lender can, subject to safe and sound banking practices, accept alternative evidence of insurance documents acceptable to the lender in order to cancel force-placed insurance. [Section-by-Section Analysis §339.7(b)(2)]
You could accept the application (with proof of payment) as Randy points out. That's what 2.a.ii. indicates You could also hold out until you get the declarations page (see 2.a.i. above).