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.
What’s your point ? Those fees are going to Western Union, not the government. As a way to collect taxes from illegals working here and paying no taxes, and collecting taxes on imported goods that obviously have no embedded US taxes, placing GOVERNMENT fees on transfers would be effective.
Sending money to your kids overseas (or having banks charge the fee when paying foreign vendors where you used your credit card while vacationing overseas) might deserve an exemption. All the data for the transactions is being kept by the financial institutions, so such details can be worked out.