Actually, putting a 20% “service fee” on ANY money transfers out of the country would be my preference.
It would cover not just these remittance by individuals, but vendors paying foreign suppliers. This would be an alternative to tariffs on goods from China, Germany, etc. All of those countries tack on a 20% VAT when American made goods are sold in their countries. That is in addition to any actual tariffs they slap on our goods. By adding a 20% service fee onto payments to those foreign suppliers, it would balance the VAT they are slapping on our stuff.
If foreign companies have the money placed into American banks and use that money to buy American goods to be exported back to their home countries, fine. Where there is no trade deficit, there would be no money transferred, but no trade deficit would mean a boost to American businesses and an increase in tax revenues from those businesses and workers.
Having sent Western Union funds to wayward children, I can tell you that you have to set up an account with that service so you can’t send anonymously. The fees are also really hefty.