Was about to poo-poo all this and say local POs or regional USPS offices can't just make things up, and it ain't a rule unless it's published in the Federal Register...
Decided to check: https://pe.usps.com/FederalRegisterNotice/Index
Published in the Federal Register March 18, 2019. New Mailing Standards for Mailpieces Containing Liquids AGENCY: Postal Service™. ACTION: Final rule. SUMMARY: The Postal Service is revising Mailing Standards of the United States Postal
Service, Domestic Mail Manual (DMM®), section 601.3.4 to clarify and supplement
the mailing standards for mailpieces containing liquids. DATES: Effective Date: March 28, 2019. FOR FURTHER INFORMATION CONTACT:Mary Collins at (202) 268-5551 or Wm. Kevin Gunther at (202) 268-7208 SUPPLEMENTARY INFORMATION:
Background The Postal Service published a notice of proposed rulemaking on July 9, 2018, (83 FR 31712-31713)
requesting public feedback on potential changes to DMM 601.3.4. The original proposed rule provided
for a 30-day comment period. At the request of the mailing industry, the comment period was
subsequently extended to September 30, 2018. During the comment period, the Postal Service received
twenty formal comments, and engaged in a number of discussions with mailers and with various members
of the mailing and hazardous materials transportation industries.
https://www.govinfo.gov/content/pkg/FR-2019-03-18/html/2019-04894.htm
600 Basic Standards for All Mailing Services
601 Mailability
* * * * *
3.0 Packaging
* * * * *
3.4 Liquids
[Revise 3.4 as follows:]
Mailers must mark the outer container of a mailpiece containing
liquid to indicate the nature of the contents (i.e., liquid), and
include orientation arrows in accordance with Publication 52, section
226. Mailers must package and mail liquids under the following
conditions:
a. Use screw-on caps with a minimum of one and one-half turns,
soldering, clips, or similar means to close primary containers
containing liquids. Do not use containers with friction-top closures
(push-down tops) except as provided in 3.4c. The use of locking rings
or similar devices are encouraged when mailing containers with
friction-top closures (push-down tops).
b. Liquids in steel pails and drums with positive closures, such as
locking rings or recessed spouts under screw-cap closures, may be
mailed without additional packaging.
c. Breakable containers including, but not limited to, those made
of glass, plastic, porcelain, and earthenware, and metal containers
with pull-tabs (pop-tops) or friction-top closures, having a capacity
of more than 4 fluid ounces must be triple-packaged according to the
following requirements:
1. Cushion the primary container(s) with absorbent material capable
of absorbing all of the liquid in the container(s) in case of breakage;
2. Place the primary container inside another sealed, leakproof
container (secondary container), such as a watertight can or plastic
bag; and
3. Use a strong and securely sealed outer mailing container durable
enough to protect the contents and withstand normal processing in
Postal Service networks.
d. As an alternative to 3.4c above, mailers may use containers
certified under the International Safe Transit Association (ISTA) Test
Procedure 3A. Mailers must, upon request, provide written test results
verifying that sample mailpieces passed each test outlined in the
standard and that no liquids were released.
* * * * *
We will publish an appropriate amendment to 39 CFR part 111 to
reflect these changes.
https://pe.usps.com/text/pub52/pub52c2_012.htm
I don't see any changes except the locking ring addition and packaging orientation arrow requirement.
My read on it is that basically it must have secure closure, be surrounded with sufficient absorbent material to soak up entire contents (yeah right), be enclosed in secondary container (Ziploc?), pkg in strong enough outer box, and have orientation arrows on 2 opposite sides of outer pkg.
Orientation arrows: draw with sharpie marker, buy labels, copy/save image below, edit/scale/print on label stock or paper)
The friction closure thing would apply to squirt type gear oil (or anything similar) bottles - the type where you pop the top open like a dish washing detergent bottle, which would pop open if pressurized, dropped etc.
One way might be wrapping non-stretchy tape (or a band of some sort) tightly over the top, around, and under the bottle a few times to keep the top down - would probably suffice to prevent accidental opening.
(wrap a paper band around first to keep tape from sticking to bottle if needed. Could also band around with tape/paper to keep main band in place better. Or just use paper in spots to keep tape wrap(s) from sticking to label parts of bottle.) More elegant retainers using wire and other parts laying about could also be devised, or commercial retainer parts are probably available.
TLDNR:
Updated rules, but pretty much change nothing. There is no prohibition of liquid shipments of more than 4oz, just the need to add orientation arrows to the pkg.
(that's my story and I'm sticking to it. If I made an error, please correct me)
