37 CFR 1.22: Fees payable in advance.

(a) Patent fees and charges payable to the United States Patent and Trademark Office are required to be paid in advance; that is, at the time of requesting any action by the Office for which a fee or charge is payable, with the exception that under § 1.53 applications for patent may be assigned a filing date without payment of the basic filing fee.

(b) All fees paid to the United States Patent and Trademark Office must be itemized in each individual application, patent, or other proceeding in such a manner that it is clear for which purpose the fees are paid. The Office may return fees that are not itemized as required by this paragraph. The provisions of § 1.5(a) do not apply to the resubmission of fees returned pursuant to this paragraph.