One you're in Shift-Command-4 or Shift-Control-Command-4 (i.e., when the cursor is a reticle), if you hit space you can select a window and it will take a screenshot of just that window, with transparent background and drop shadow.
Yes, I found that out from Mac Help in High Sierra; that's what I was using
repeatedly on my
test Customs Declaration (addressed to the current business of one of the two Australians who were my direct bosses when I worked in Teaneck NJ 1999-2004) when I got the "camera clicks"—but nothing in the clipboard. I eventually found out from Mac Help about Apple's Grab utility, which gave me a .PDF—instead of a .PNG— on the desktop.
I have not yet attached this test .PDF in an e-mail to ShipStation, because I'm not sure whether their User Success Specialist Mia G. knows how to open an e-mail attachment. Kari G., the Mgr. User Support who is Mia G.'s boss, replied to my last e-mail to Mia G.
Regarding the request to reply to your emails and include a copy of your previous reply, I'm afraid that the software we utilize in order to reply to your requests does not allow for this option. I certainly apologize for that.
ShipStation is a software company with many "partners" besides PayPal; my impression is that they hire their User Success Specialists straight from the Midwestern cornfields and strictly choreograph their interactions.
Therefore I don't believe Mia G. when she wrote
I can confirm the system will not allow a harmonization code that is under 6 characters as it is a carrier requirement for the code to be 6-15 characters. After some testing on my side, I received an error message after entering 4904, as intended.
You can create a test label on your side and later void the label for a refund. It is important to test these features on your side to ensure you are no longer able to enter an HS code that is less than 6 characters and to see if the system will allow a code with decimal points.
For one thing, I had previously sent 3 packages to my customer in Western Australia, and the Harmonization code I entered was 4904. It's always possible that ShipStation's developers have
very recently added an error message, but the requirement should be
7–
13 characters—because the 5th through 7th characters should be a decimal point followed by a 2-digit sub-category such as .00 . For another thing, early on 22 May I received an unrelated order from a customer in Yorkshire UK; when I immediately ordered the pre-paid label through ShipStation's application, I
successfully entered the Harmonization code 4904.00.00.00 to be on the safe side. That implies a
test is no longer necessary.
I have not
yet informed ShipStation that I created a label for this unrelated order, because of a discovered workflow that is unparalleled in my 40 years as a (now-retired) commercial applications programmer. When a label for an international order is created, ShipStation gives it a South Hackensack NJ address and a USPS tracking code—but prints a Customs tracking code and the order's ultimate destination at the bottom under "Electronic Customs Service" and the logo of sub-contractor GlobalPost. USPS tracking shows that GlobalPost picks up the package in South Hackensack and transports it to Lyndhurst NJ—where it prints a new label with the ultimate destination and Customs number and pastes it over the existing label. The package is then transported back to South Hackensack, and from there to JFK Airport in Queens NYC.

(For those of you unfamiliar with NYC-area geography, New Jersey is West of NYC's Hudson River boundary, but Queens is the farthest-East borough of NYC across a Long Island Bay estuary mis-named the
East River.) I don't want ShipStation messing with the progress of this package until it has safely passed through UK Customs, because I think ShipStation's capable of interference merely to make me a liar.
Sorry for the digression in the preceding 3 paragraphs, but IMHO it's generally-useful public information. I'll drop the subject now, unless my latest package is rejected by UK Customs.
Edit: Clarifications in next-to-last paragraph.