Maybe one way to do this is to put the responsibility on the test taker to go back into ULS after passing a test to pay the fee. The test taker should already have an FRN before they arrive to take a test.
The process for VE teams and VECs wouldn't change as they would not be responsible for collecting large sums of money.
The FCC computer looks to see if the license fee has been paid before issuing a new/upgraded license. The person has "X" number of days to pay the fee or the new/upgrade license the VEC entered in the computer is voided and the license is never issued.
Not sure how you might police that. There are people who will operate with their new upgraded privledges anyway after the period expires. Perhaps the fee will pay for FCC enforecement?
If this NPRM goes through, the number of licenses WILL decline. There is a large number of people with licenses who have no intention of using it, they have lost interest, or they got it because somebody told them it would be a good thing to have for another activity and have no real interest in amateur radio. Those people will let their licenses lapse. Those of us who are left may be more likely to get on the air because we have a little more invested in amateur radio.
73,
Rich, KA4GFY