Amazon FBA Storage Fees

Hi

I wondered if some one can help me, rather than writing into seller support and not getting a straight simple answer.

I am looking at Amazon FBA storage fees, and a product that I have got that I want to send into them, measures and falls into the category to be charged 0.43p per unit per month.

Please can someone confirm to me, a couple of things

Will I be charged 0.43p per unit thats in storage for 30days or
will i be charged on a pro rata basis, for example if my item goes into the fulfilment centre on 1st of month and sells on the 10th would I be charged 1/3 of the per month storage cost because the unit has only been in the fulfilment centre for 10 out of the 30 days that they charge 0.43p per unit on?

many thanks