27 C.F.R. § 19.238 – Payment by mail or courier.

(a) Payment by mail. The proprietor must file each return on form TTBF 5000.24 in accordance with the instructions printed on the form. If the proprietor submits the return by U.S. mail, the official postmark of the U.S. Postal Service stamped on the cover in which the return is mailed will be considered to be the date of delivery of the return and also the remittance, if included. If the postmark on the cover is illegible, the proprietor will bear the burden of proving when the postmark was made. If the proprietor sends the return with or without remittance by registered mail or certified mail, the date of registry, or the date of the postmark on the sender's postal receipt for certified mail, will be treated as the date of delivery of the return and also of the remittance, if included.
(b) Payment by courier or other private delivery service. A proprietor may send a return, with or without remittance, by courier or other private delivery service. If the proprietor sends the return with or without remittance with a courier or private delivery service that is available to the general public and that is at least as timely and reliable as the U.S. mail, and the delivery service has tracking and tracing procedures for its deliveries, TTB will consider the date of tender to the delivery service as recorded in the tracking and tracing record for the parcel as the date of delivery. If the proprietor sends the return, with or without remittance, by courier or other private delivery service that does not meet the above requirements, the actual date of delivery to TTB will be treated as the date of delivery of the return and also of the remittance, if included.
(26 U.S.C. 6302)