Just did a test run for my 1st on-line customs form--because the new "hand filled out" forms are worse! It seemed to print quickly and not a lot of ink on my HP laser printer*. So maybe the PO assumed that's what everyone has.
*A tip: I carefully highlighted each page and copy/pasted onto a Word document as a bitmap. I was able to get 2 pages of the form on each piece of 8.5 by 11 paper, so used 2 1/2 pages instead of 5. AND that may be why it was quicker and cheaper to print then the actual USPS web application screen.
I don't send as many packages as others here; if someone else tries this, let us know if it works better than printing directly from the web application.